quak files COLLECTION fetches that collection's files alone (closes #217)

`quak files COLLECTION` used to wait for a full refresh, which on an
empty cache fetched the files of every collection before it listed one.
It now reads the collection list, then calls the new
`Library.refreshCollection()`. That brings one collection up to date in
the store as a refresh would, and saves it. It runs in the same slot as
a refresh, so the two never run at the same time, and `close()` waits
for it. It never stores a collection that a refresh has already dropped.
It leaves the refresh cursor alone. `open()` therefore awaits the first
refresh on a cache whose cursor is still 0. `quak files` with no
argument still waits for a refresh.

Model: opus-5-5

README.md

@@ -1099,11 +1099,12 @@ against a fake of the Ente HTTP API, and `yarn test` verifies them.
 loop, and resolves to a `Library`. On an empty cache it awaits the first
 refresh, so it opens onto the account's data whenever the server is reachable.
 It also awaits it on a cache that holds only what `lib.refreshCollection()`
-stored. If that refresh fails, it opens onto what the cache holds and records
+stored, so it never serves one collection as the whole account. If that refresh
-the error in `lib.status()`. On any other cache it returns immediately and
+fails, it opens onto what the cache holds and records the error in
-refreshes in the background, so an unreachable server does not block opening.
+`lib.status()`. On any other cache it returns immediately and refreshes in the
-With `refreshOnOpen: false` it starts no refresh and returns at once onto what
+background, so an unreachable server does not block opening. With
-the cache holds, empty or not; a refresh then starts only when `fresh()` or
+`refreshOnOpen: false` it starts no refresh and returns at once onto what the
+cache holds, empty or not; a refresh then starts only when `fresh()` or
 `lib.backup()` asks, or the background timer fires. The CLI's read commands open
 it so (see "CLI surface").
 

src/library/index.ts

@@ -944,10 +944,10 @@ export class Library {
         if (changed) this.unsaved = true;
 
         // Reproject and notify subscribers of the delta. This tracks RAM (what
-        // reads see), so it fires whether or not the save below succeeds. A
+        // reads see), so it fires whether or not the save below succeeds; a
-        // failed save rejects the caller, and a refresh also records it in
+        // save failure surfaces separately through `status().lastError`.
-        // `status().lastError`. `lastRecords` advances every changed refresh so
+        // `lastRecords` advances every changed refresh so the next diff is
-        // the next diff is against current state.
+        // against current state.
         if (changed) {
             const next = this.deriveNow();
             if (this.subscribers.size > 0) {
@@ -964,9 +964,10 @@ export class Library {
 
         // Persist whenever RAM holds changes disk has not accepted — including
         // changes an earlier cycle staged whose save failed. `unsaved` clears
-        // only once a save lands, so a later commit retries a failed save even
+        // only once a save lands, so a save failure both stays visible through
-        // when it changes nothing. A failed save rejects the caller, and a
+        // `status().lastError` (the throw below records it) and keeps being
-        // refresh also records it in `status().lastError`.
+        // retried, instead of a later empty refresh silently clearing it while
+        // the on-disk cache is still behind RAM.
         if (this.unsaved) {
             await this.store.save();
             this.unsaved = false;