feat(core): adopt feedpak_version — read on load + stamp on manifest writes (spec §4) (#530)

Core never read or emitted the manifest `feedpak_version` field. Adopt it:

- sloppak.py: `FEEDPAK_VERSION = "1.2.0"` constant (the format version this build
  targets); `LoadedSloppak.feedpak_version` read from the manifest on load
  (string, else None for legacy/absent).
- Stamp the version on the two core manifest-rewrite paths, without downgrading
  an existing (possibly higher) declared version:
  - gp2notation: `setdefault` before its notation-add rewrite.
  - songmeta: opportunistically when a metadata field is supplied (gated on the
    existing `dirty` flag, so never a standalone rewrite).

Core has no create-from-scratch path (RS-free repo) — the editor plugin's
create-mode save stamping FEEDPAK_VERSION is a follow-up in that repo. Internal
"sloppak" naming is intentionally left as-is (a rename is out of scope / risky).

Codex-reviewed: no P1/P2. +6 tests (read present/absent/non-string; metadata-write
stamp-when-absent / preserve-existing / no-op-no-stamp) + updated the gp2notation
key-order test for the appended version. 197 sloppak/songmeta/gp2notation tests pass.

Closes #527. Part of got-feedback/feedback#334.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/gp2notation.py

@@ -522,6 +522,10 @@ def attach_notation_to_sloppak(sloppak_dir: str | Path, arr_id: str, payload: di
         json.dumps(payload, separators=(",", ":")), encoding="utf-8"
     )
     entry["notation"] = filename
+    # Stamp the format version while we're rewriting the manifest (spec §4),
+    # without downgrading an existing (possibly higher) declared version.
+    from sloppak import FEEDPAK_VERSION
+    manifest.setdefault("feedpak_version", FEEDPAK_VERSION)
     manifest_path.write_text(
         yaml.safe_dump(manifest, sort_keys=False, allow_unicode=True),
         encoding="utf-8",