fix(brainfs): bound the read in ReadRegularMax; honest test names (#157)

Review round 1 on PR #165:

- ReadRegularMax checked the size on the opened handle, then read with an
  unbounded io.ReadAll, so a file growing after the check returned past the
  limit. The read now goes through io.LimitReader(max+1). A test seam grows
  the file between the check and the read and counts bytes consumed.
- New: ReadScoped returns ErrTooLarge for a note over maxNoteBytes; red if
  readContained drops the limit.
- notes.Target carries the brain root instead of deriving it by TrimSuffix.
- Rule 9: reader tests renamed to <Reader>RefusesSymlinksAndSurvivesSwapRaces;
  ReadNote test has a positive control; brainfstest comment states what the
  harness observes and what it does not.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/brainfs/brainfs.go

@@ -108,9 +108,28 @@ func (b *Brain) readOpenedMax(rel string, max int64) ([]byte, error) {
 	if max >= 0 && hfi.Size() > max {
 		return nil, fmt.Errorf("%s is %d bytes (limit %d): %w", rel, hfi.Size(), max, ErrTooLarge)
 	}
-	return io.ReadAll(f)
+	var r io.Reader = f
+	if wrapAfterStat != nil {
+		r = wrapAfterStat(r)
+	}
+	if max < 0 {
+		return io.ReadAll(r)
+	}
+	// The handle's size can grow after the check; bound the read itself.
+	data, err := io.ReadAll(io.LimitReader(r, max+1))
+	if err != nil {
+		return nil, err
+	}
+	if int64(len(data)) > max {
+		return nil, fmt.Errorf("%s grew past %d bytes while read: %w", rel, max, ErrTooLarge)
+	}
+	return data, nil
 }
 
+// wrapAfterStat is a test seam: it runs between the size check and the read,
+// and wraps the reader the read consumes.
+var wrapAfterStat func(io.Reader) io.Reader
+
 // WalkRegular walks a brain-relative directory through the root and calls fn
 // for each regular file (directories are descended; symlinks, FIFOs and
 // devices are skipped, never followed). An entry error is passed to onErr and