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/extract/extract.go

@@ -2,6 +2,7 @@
 package extract
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -50,6 +51,18 @@ func fileExt(path string) string {
 // external tool (pdf, docmark) are copied to a temp file from the handle
 // opened through the root, then extracted from the copy.
 func TextIn(r *os.Root, rel string) (string, error) {
+	return TextInMax(r, rel, DefaultMaxBytes)
+}
+
+// DefaultMaxBytes bounds what TextIn reads or copies from one file.
+const DefaultMaxBytes = 64 << 20
+
+// ErrTooLarge reports a file that exceeded the read cap while being read: the
+// size seen at an earlier Lstat is not a bound, the file may have grown.
+var ErrTooLarge = errors.New("file exceeds the read cap")
+
+// TextInMax is TextIn with an explicit cap on the bytes read (hyperguild#198).
+func TextInMax(r *os.Root, rel string, max int64) (string, error) {
 	fi, err := r.Lstat(rel)
 	if err != nil {
 		return "", fmt.Errorf("stat %s: %w", rel, err)
@@ -65,10 +78,13 @@ func TextIn(r *os.Root, rel string) (string, error) {
 	ext := strings.ToLower(fileExt(rel))
 	switch ext {
 	case ".md", ".txt":
-		b, err := io.ReadAll(f)
+		b, err := io.ReadAll(io.LimitReader(f, max+1))
 		if err != nil {
 			return "", fmt.Errorf("read %s: %w", rel, err)
 		}
+		if int64(len(b)) > max {
+			return "", fmt.Errorf("%s: %w (%d bytes)", rel, ErrTooLarge, max)
+		}
 		return string(b), nil
 	case ".pdf", ".docx", ".xlsx", ".pptx", ".png", ".jpg", ".jpeg":
 		tmp, err := os.CreateTemp("", "ingest-*"+ext)
@@ -76,10 +92,15 @@ func TextIn(r *os.Root, rel string) (string, error) {
 			return "", fmt.Errorf("temp copy: %w", err)
 		}
 		defer func() { _ = os.Remove(tmp.Name()) }()
-		if _, err := io.Copy(tmp, f); err != nil {
+		n, err := io.Copy(tmp, io.LimitReader(f, max+1))
+		if err != nil {
 			_ = tmp.Close()
 			return "", fmt.Errorf("copy %s: %w", rel, err)
 		}
+		if n > max {
+			_ = tmp.Close()
+			return "", fmt.Errorf("%s: %w (%d bytes)", rel, ErrTooLarge, max)
+		}
 		if err := tmp.Close(); err != nil {
 			return "", fmt.Errorf("copy %s: %w", rel, err)
 		}