fix(ingestion): close the PR #156 review findings on raw/ reads

- Reverting the ingest call sites to extract.Text(path) stayed green;
  a test now fails if anything outside internal/extract calls
  extract.Text( (only TextIn, through an os.Root, may be used).
- The watcher symlink test passed for a watcher that ingested nothing,
  or stopped at the first symlink (SkipAll: one synced link halting all
  ingestion). It now plants a real note after the symlink and asserts
  exactly one ingest, and the symlinked-raw refusal has its own test.
- A FIFO swapped in between Lstat and Open hung open(2) forever.
  extract.OpenRegular opens non-blocking and checks the type on the
  handle; readRawNote, TextIn and the watcher's archive copy use it.
- The symlinked-root test covers pending, promote, reject, resolve and
  ReadIngestFile, and is named for exactly those.
- The hard-link residual is documented where it is accepted.

Out of scope, filed: #157 (symlinks elsewhere in the Syncthing-shared
brain dir reach search and other readers), #158 (writes under raw/
follow symlinks).

Refs #153

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

ingestion/internal/watcher/watcher.go

@@ -238,7 +238,7 @@ func copyFromRoot(root *os.Root, rel, dst string) error {
 	if !fi.Mode().IsRegular() {
 		return fmt.Errorf("%s: not a regular file", rel)
 	}
-	in, err := root.Open(rel)
+	in, _, err := extract.OpenRegular(root, rel)
 	if err != nil {
 		return err
 	}