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/notes/path.go

@@ -58,8 +58,9 @@ var (
 
 // Target is a resolved, contained path.
 type Target struct {
-	Rel string // brain-relative, cleaned
-	Abs string // absolute, symlink-checked
+	Rel   string // brain-relative, cleaned
+	Abs   string // absolute, symlink-checked
+	Brain string // the brain root Rel is relative to
 }
 
 // Resolve canonicalises path, then checks it against the scope's allowlist,
@@ -109,7 +110,7 @@ func Resolve(brainDir, path string, scope Scope) (Target, error) {
 		return Target{}, err
 	}
 
-	return Target{Rel: clean, Abs: abs}, nil
+	return Target{Rel: clean, Abs: abs, Brain: brainDir}, nil
 }
 
 // allow enforces the scope's allowlist. The two scopes are DISJOINT, not