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>

tests/test_gp2notation.py

@@ -502,8 +502,12 @@ def test_attach_notation_to_sloppak(tmp_path):
     entries = {e["id"]: e for e in rewritten["arrangements"]}
     assert entries["keys"]["notation"] == "notation_keys.json"
     assert "notation" not in entries["lead"]
-    # Key order preserved (sort_keys=False round-trip).
-    assert list(rewritten.keys()) == ["title", "artist", "arrangements", "stems"]
+    # Original key order preserved (sort_keys=False round-trip); the manifest
+    # rewrite also stamps feedpak_version (spec §4), appended at the end.
+    assert list(rewritten.keys()) == [
+        "title", "artist", "arrangements", "stems", "feedpak_version"]
+    from sloppak import FEEDPAK_VERSION
+    assert rewritten["feedpak_version"] == FEEDPAK_VERSION
 
 
 def test_attach_notation_unknown_arrangement_raises(tmp_path):