README.md rewritten sentence by sentence (closes #219)

The old text served as the list of what a reader needs covered. The new
text leads with what quak is, what it is for and how to use it. Every
command, option, exit code and file layout follows in reference sections,
then the library reference, then design, entrypoints and development.
Every statement was checked against the code on next; the places where
the old text disagreed with it are listed in the pull request. No sentence
runs past about 30 words, every table is now a list, and no issue links or
project history remain in text meant for users. Two code comments that
named README sections now name the new ones.

Model: fable-5-1

README.md

@@ -314,8 +314,8 @@ refused, because both files would land on it.
 #### `quak get-thumb <fileID> [--out <path>]`
 
 Downloads and decrypts one file's thumbnail, the small JPEG Ente shows in a
-grid. It works as `get` does. Without `--out`, the name is `thumb_` followed by
-the title.
+grid. It writes one file, also for a live photo. Without `--out`, the name is
+`thumb_` followed by the title. In all else it works as `get` does.
 
 #### `quak backup <dir> [--json] [--verify]`
 
@@ -362,7 +362,8 @@ Writes the metadata quak keeps about the account, its collections and its files
 as JSON files under `<dir>`. The layout is described under "The metadata dump
 directory". Photo content is not written. With `--exif`, or its alias `--all`,
 the command also fetches each file's original and records the image's own
-metadata from it (see "The metadata dump directory").
+metadata from it (see "The metadata dump directory"). Each original it fetches
+stays in the cache.
 
 Ente's apps compute machine-learning data for each photo, such as the faces in
 it (see "The JSON beside each original"). The command fetches it in requests of
@@ -377,9 +378,9 @@ word, and does not change the exit status.
 
 #### `quak helper list-missing-thumbnails [--json]`
 
-Finds the files whose thumbnail is missing on the server. It downloads every
-thumbnail in the account, one collection at a time, newest collection first, so
-on a large account it takes a while. A thumbnail counts as missing when the
+Finds the files whose thumbnail is missing on the server. It downloads the
+thumbnail of every file you own, one collection at a time, newest updated first,
+so on a large account it takes a while. A thumbnail counts as missing when the
 server answers with an empty body or with HTTP 404. Any other failure is logged
 as `[<collection>] Could not check <title>: <reason> (not reported as missing)`,
 and the file is left out of the list. The command exits 0 even when some files
@@ -477,16 +478,16 @@ before any request is sent. The command logs `Invalid file ID: <arg>` or
 
 ### What quak prints
 
-stdout holds only a command's output: the listing, the JSON, or nothing. The
-prompts of `login` go to stdout too. Every other line goes to stderr as a log
-line at one of six levels. From the quietest, the levels are fatal, error,
-warning, info, debug and trace. A run prints the lines at its own level and at
-every quieter one. The level is info unless `-v`, `-vv` or `-q` says otherwise.
+stdout holds a command's output, such as the listing or the JSON, and the
+prompts of `login`. Every other line goes to stderr as a log line at one of six
+levels. From the quietest, the levels are fatal, error, warning, info, debug and
+trace. A run prints the lines at its own level and at every quieter one. The
+level is info unless `-v`, `-vv` or `-q` says otherwise.
 
-The one exception is a command line quak cannot parse, such as a missing
-argument, an unknown option or `-q` with `-v`. quak refuses it with one
-`error: …` line that is not a log line, and exits 1. `-q` does not hide that
-line. `quak` with no command prints the usage on stderr and exits 1.
+Two things on stderr are not log lines. A command line quak cannot parse, such
+as a missing argument, an unknown option or `-q` with `-v`, is refused with an
+`error: …` message, and quak exits 1. `-q` does not hide the message. `quak`
+with no command prints the usage on stderr and exits 1.
 
 `yarn quak` prints yarn's own lines on stdout as well. To pipe a command's
 output, run `yarn --silent quak files --json` or
@@ -783,10 +784,11 @@ goes away.
 Each run removes every symlink in a collection's directory that points into the
 date folders and is not one of its current links, yours included. A file or
 symlink of yours at the name of one of its links is replaced by the link. A
-directory at that name stays, and the file goes into `failures.json`. Anything
-else you put under `collections/` stays. A run removes the directory and JSON of
-a collection that was deleted or renamed, once the directory holds nothing else.
-One that still holds something of yours stays, with its JSON.
+directory of yours at that name stays, and the photo whose link it blocks goes
+into `failures.json`. Anything else you put under `collections/` stays. A run
+removes the directory and JSON of a collection that was deleted or renamed, once
+the directory holds nothing else. One that still holds something of yours stays,
+with its JSON.
 
 A collection whose directory or JSON cannot be written does not stop the run.
 For example, a file you put there may already have the directory's name. The run
@@ -831,7 +833,7 @@ Each entry holds:
 
 - `fileID` and `title`;
 - `classification`: the kind of failure;
-- `attempts`: how many runs have tried the file;
+- `attempts`: how many runs in a row have failed the file;
 - `lastTriedAt`: when it was last tried, as a Unix time in milliseconds;
 - `error`: the error, as quak printed it.
 
@@ -1183,11 +1185,11 @@ progress of that one file.
   keyed by tag name, each as exifreader decodes it with its `id`, `value`,
   `description` and `computed` value. For example, `Make` is
   `{ id: 271, value: ["Example"], description: "Example", computed: "Example" }`.
-  A tag exifreader has no name for is keyed `undefined-<tag number>`. The tags
-  of the thumbnail embedded in the file are under `Thumbnail`, so they cannot
-  hide the main image's tags of the same name. The thumbnail image itself is
-  left out. Any file exifreader cannot read gives `{}`, and a video gives `{}`
-  without being downloaded.
+  A tag that exifreader has no name for is keyed `undefined-<tag number>`. The
+  tags of the thumbnail embedded in the file are under `Thumbnail`, so they
+  cannot hide the main image's tags of the same name. The thumbnail image itself
+  is left out. Any file exifreader cannot read gives `{}`, and a video gives
+  `{}` without being downloaded.
 - `await photo.make(opts?)` returns one common EXIF field, picked from the tags
   `exif()` returns and typed as in `PhotoExif`, or `undefined` when the file
   lacks it. These methods do the same for their fields:
@@ -1307,8 +1309,9 @@ Only the requests named above go through the pools. A refresh,
 with
 `new RequestPools({ metadataConcurrency, contentConcurrency, thumbnailConcurrency })`
 and pass it as `pools`. Each pool serves on-demand work ahead of background
-work. A task submitted under a key that is still pending is not run a second
-time. An idle pool never lends its slots to a busy one.
+work. Two tasks given to `BoundedPool.run()` with the same `key` run once while
+the first is pending, and both get its result. An idle pool never lends its
+slots to a busy one.
 
 `lib.backup()` works on as many originals at once as the content pool allows, 5
 by default. `quak backup` uses the default. The limit covers everything it does
@@ -1402,13 +1405,13 @@ that rule although they look like connect-time failures. On Linux they are also
 delivered on an established socket, after the request may have been written.
 They stay retryable for requests that change nothing. Bun's `ConnectionRefused`
 and `FailedToOpenSocket` are treated the same way: a request that changes
-nothing is retried on them, and a `POST` or `PUT` is not replayed on them. When
-a connection bun kept open from an earlier request closes before any answer, bun
-sends the request again on a new connection. If that one fails, bun gives one of
-these codes although the server may already have the request. The upload of a
-thumbnail to its presigned URL is exempt from the rule. It stores one whole
-object at one key in one request, so replaying it has no partial state to
-damage.
+nothing is retried on them, and a `POST` or `PUT` is not replayed on them. Bun
+reuses a connection it kept open from an earlier request. When that connection
+closes before any answer, bun sends the request again on a new one. If that one
+fails, bun gives one of these codes although the server may already have the
+request. The upload of a thumbnail to its presigned URL is exempt from the rule.
+It stores one whole object at one key in one request, so replaying it has no
+partial state to damage.
 
 A download is retried as a whole: the request, the reading of the body and the
 decryption. A socket reset after the response headers have arrived surfaces in
@@ -1446,11 +1449,11 @@ working client from that snapshot without logging in again. It checks every
 field and each key's length first, and throws an error naming the bad field.
 
 `Client.fromJSON` works only once libsodium, the cryptography library quak uses,
-is loaded. Only `Client.login` loads it. A program that restores a session in a
-new process first runs `await sodium.ready`, with `sodium` imported from
-`libsodium-wrappers-sumo`, which quak depends on. The exported `beginLogin`,
-`unwrapAuth`, `decryptCollection`, `decryptFile`, `downloadFile` and
-`downloadThumbnail` need the same.
+is loaded. `Client.login` loads it, and `Client.fromJSON` does not. A program
+that restores a session in a new process first runs `await sodium.ready`, with
+`sodium` imported from `libsodium-wrappers-sumo`, which quak depends on. The
+exported `beginLogin`, `unwrapAuth`, `decryptCollection`, `decryptFile`,
+`downloadFile` and `downloadThumbnail` need the same.
 
 `client.logout()` clears the token and zeroes the key buffers in place. After
 it, every other method on that client throws. It does not contact the server, so
@@ -1907,7 +1910,9 @@ These apply to people and to coding agents alike.
 - Not pinned:
     - bun, which `make build-bin` takes from `nix-shell -p bun`;
     - `make` and `git`, which `script/bootstrap` installs from the system
-      package manager when they are missing, as every `Dockerfile` stage does.
+      package manager when they are missing, as every `Dockerfile` stage does;
+    - `bash` and `curl`, which `script/bootstrap` installs the same way when it
+      has to install Node.
 - Follow the workflow above: branches off `next`, tests first and committed
   failing, and `next` merged into `main` by sneak alone.
 - Before every commit, `make lint` and `make fmt-check` must pass. `make lint`