Rewrite README.md sentence by sentence (closes #327)

Rewrites README.md from scratch against the code on next, for
#327: what pixa is and how to
use it, deployment, design, the settings reference, then development
and testing. The Configuration list keeps one item per setting, as
TestReadmeNamesEverySetting reads it; the test is unchanged. The
statements where the old text disagreed with the code are listed in
the PR body.

Model: fable-5-1

TODO.md

@@ -31,8 +31,9 @@ P2: security: per-origin rate limiting
 # Completed Steps
 
 - 2026-10-27 the reconciliation pass skips a directory it cannot list and a file
-  whose info it cannot read, and goes on (closes #343). It returned the error,
-  so one such entry stopped every pass before its later checks.
+  whose info it cannot read, and goes on (closes #343). It stopped before its
+  later checks at a directory it could not list, or at a source or variant file
+  whose info it could not read.
     - It logs the warning "skipped unreadable entry in cache directory", with
       the path and the error.
     - A file renamed or removed after its directory was listed gets no warning.

internal/imgcache/eviction_internal_test.go

@@ -819,10 +819,11 @@ func openDirectoryNearPathLimit(t *testing.T, dir string) *os.Root {
 	return root
 }
 
-// makeEntriesTooLongToRead makes a directory and a file in the directory
-// that openDirectoryNearPathLimit opens inside dir. Even the root user can
-// then neither list that directory nor read the file's info. It returns
-// their two paths.
+// makeEntriesTooLongToRead makes a new directory and a new file in the
+// deepest directory of the chain that openDirectoryNearPathLimit makes
+// inside dir. Even the root user can then neither list the new directory
+// nor read the new file's info. It returns the paths of the new directory
+// and the new file.
 func makeEntriesTooLongToRead(t *testing.T, dir string) []string {
 	t.Helper()
 
@@ -847,9 +848,10 @@ func makeEntriesTooLongToRead(t *testing.T, dir string) []string {
 	}
 }
 
-// makeTempFileTooLongToRead makes a temp file in the directory that
-// openDirectoryNearPathLimit opens inside dir. Even the root user then
-// cannot read its info. It returns its path.
+// makeTempFileTooLongToRead makes a temp file in the deepest directory of
+// the chain that openDirectoryNearPathLimit makes inside dir. Even the
+// root user then cannot read the temp file's info. It returns the temp
+// file's path.
 func makeTempFileTooLongToRead(t *testing.T, dir string) string {
 	t.Helper()
 
@@ -864,6 +866,25 @@ func makeTempFileTooLongToRead(t *testing.T, dir string) string {
 	return filepath.Join(deepest.Name(), name)
 }
 
+// assertSkippedEntryWarning checks that logs hold the warning "skipped
+// unreadable entry in cache directory" naming path, with the error
+// ENAMETOOLONG for that path.
+func assertSkippedEntryWarning(t *testing.T, logs, path string) {
+	t.Helper()
+
+	for line := range strings.Lines(logs) {
+		if strings.Contains(line, `"level":"WARN"`) &&
+			strings.Contains(line, `"msg":"skipped unreadable entry in cache directory"`) &&
+			strings.Contains(line, `"path":"`+path+`"`) &&
+			strings.Contains(line, path+": "+syscall.ENAMETOOLONG.Error()) {
+			return
+		}
+	}
+
+	t.Errorf("no warning %q names %s with the error %q",
+		"skipped unreadable entry in cache directory", path, syscall.ENAMETOOLONG)
+}
+
 // TestReconciliationGoesOnPastEntriesItCannotRead puts, in each storage
 // directory, a directory the pass cannot list and a file whose info it
 // cannot read. The pass walks each directory in name order, so it reaches
@@ -915,11 +936,7 @@ func TestReconciliationGoesOnPastEntriesItCannotRead(t *testing.T) {
 	logs := reconcileAsAtStartup(t, cache)
 
 	for _, path := range unreadable {
-		assertWarningNames(t, logs, path)
-
-		if !strings.Contains(logs, path+": "+syscall.ENAMETOOLONG.Error()) {
-			t.Errorf("no log line gives the error reading %s", path)
-		}
+		assertSkippedEntryWarning(t, logs, path)
 	}
 
 	rowQuery := `SELECT COUNT(*) FROM variant_content WHERE cache_key = ?`
@@ -948,11 +965,11 @@ func TestReconciliationGoesOnPastEntriesItCannotRead(t *testing.T) {
 	}
 }
 
-// TestReconciliationWarnsOfTempFilesItCannotRead puts, in each storage
-// directory, a temp file whose info the pass cannot read. The pass must
-// log a warning naming each one's path and the error, as it does for any
-// other file whose info it cannot read.
-func TestReconciliationWarnsOfTempFilesItCannotRead(t *testing.T) {
+// TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead puts, in each
+// storage directory, a temp file whose info the pass cannot read. The
+// pass must log a warning naming each one's path and the error, as it
+// does for any other file whose info it cannot read.
+func TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
@@ -965,11 +982,7 @@ func TestReconciliationWarnsOfTempFilesItCannotRead(t *testing.T) {
 	logs := reconcileAsAtStartup(t, cache)
 
 	for _, path := range unreadable {
-		assertWarningNames(t, logs, path)
-
-		if !strings.Contains(logs, path+": "+syscall.ENAMETOOLONG.Error()) {
-			t.Errorf("no log line gives the error reading %s", path)
-		}
+		assertSkippedEntryWarning(t, logs, path)
 	}
 }