fix(watcher): unique archive copies, read caps at the read, stale markers (review of #210)

From the independent review of #210:
- F1: processed/ and failed/ copies are created O_EXCL with -2,-3 suffixes via
  createUnique; same-named notes from different folders no longer replace one
  another's only surviving copy
- F2: extract.TextInMax / readAllMax cap the bytes actually read (a note can
  grow after the Lstat); extract.TextIn gets a 64 MiB default cap
- F3: a regular marker older than its note describes an earlier note of the
  same name; it no longer hides the new one (warn + remove); non-regular
  markers still count as done (no writes through links)
- F4: the planted-marker-link test is renamed to what it observes
Each has a test that fails without it; the read-cap test uses an endless reader
that errors at 8 MiB so a regression fails instead of exhausting memory.

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

ingestion/internal/watcher/park.go

@@ -20,6 +20,13 @@ import (
 // root, never replacing a parked note: on a name clash it tries -2, -3, ...
 // A needs-review that is a symlink, or any non-directory, is refused.
 func createParked(brainDir, filename string, data []byte) error {
+	return createUnique(brainDir, "raw/needs-review", filename, data)
+}
+
+// createUnique writes data to dirRel/filename through the brain root, never
+// replacing an existing file: on a name clash it tries -2, -3, ... A dirRel that
+// is a symlink, or any non-directory, is refused.
+func createUnique(brainDir, dirRel, filename string, data []byte) error {
 	bfs, err := brainfs.Open(brainDir)
 	if err != nil {
 		return err
@@ -32,7 +39,7 @@ func createParked(brainDir, filename string, data []byte) error {
 		if n > 1 {
 			name = fmt.Sprintf("%s-%d%s", stem, n, ext)
 		}
-		err := bfs.CreateRegular("raw/needs-review/"+name, data)
+		err := bfs.CreateRegular(dirRel+"/"+name, data)
 		if err == nil {
 			return nil
 		}
@@ -40,5 +47,5 @@ func createParked(brainDir, filename string, data []byte) error {
 			return err
 		}
 	}
-	return fmt.Errorf("no free name for %q in raw/needs-review", filename)
+	return fmt.Errorf("no free name for %q in %s", filename, dirRel)
 }