feat(core): read .jsonc data files (strip C-style comments) (feedpak-spec §8)

feedpak-spec §8 (FEP #3 / PR #13) allows .jsonc files (JSON with C-style
// line and /* */ block comments) anywhere .json is specified. A Reader MUST
strip comments before parsing. Core's sloppak/feedpak readers parsed every
side file with bare json.loads, so a .jsonc arrangement / notation / drum_tab
/ song_timeline / lyrics / keys would fail to load.

- lib/jsonc.py (new): shared parse_jsonc(text) + load_json(path). String-aware
  regex (mirrors the spec reference validator in feedpak-spec/tools/validate.py)
  — keeps comment-like text inside JSON string literals. load_json auto-detects
  .jsonc by suffix; plain .json goes straight through json.loads.
- lib/sloppak.py: import load_json; replace the 6 json.loads(...read_text...)
  side-file read sites (arrangement, notation, drum_tab, song_timeline,
  lyrics, keys) with load_json(<path>). Removed the now-unused `import json`.
- scripts/lift_keys_notation.py: import load_json; replace the 3 read sites
  (song_timeline, arrangement beats fallback, arrangement lift). `import json`
  stays (json.dumps write at the notation sidecar emit).

Additive (MINOR) change: older readers parse .jsonc as plain JSON and ignore
comments via the spec's forward-compatibility rules, so no existing pack needs
regeneration.

Tests: tests/test_sloppak_jsonc_load.py (16 tests) — parse_jsonc unit cases
(line/block/multiline/string-boundary/malformed/plain), and end-to-end loads
for all 6 side-file types via .jsonc with comments, plus the lift helper
reading .jsonc song_timeline + .jsonc arrangement beats, plus the
string-boundary preservation rule through the full loader. 122 sloppak/lift
tests pass.

scripts/lift_keys_notation.py

@@ -60,6 +60,8 @@ import yaml  # noqa: E402
 
 import notation as notation_mod  # noqa: E402, F401  (re-exported for tests)
 
+from jsonc import load_json  # noqa: E402
+
 # The wire→notation heuristic core lives in ``lib/notation_lift.py`` so it can
 # be reused in-process (e.g. by the Arrangement Editor's notation save path)
 # rather than being copy-pasted out of this one-time CLI. Re-exported here so
@@ -114,7 +116,7 @@ def _load_song_beats(pak: Path, manifest: dict) -> list[dict]:
         st_path = _safe_child(pak, st_rel)
         if st_path is not None and st_path.is_file():
             try:
-                data = json.loads(st_path.read_text(encoding="utf-8"))
+                data = load_json(st_path)
                 # An empty beats list is not an authoritative timeline — fall
                 # through to the arrangement JSONs rather than ending up with
                 # zero downbeats and skipping the whole sloppak.
@@ -134,7 +136,7 @@ def _load_song_beats(pak: Path, manifest: dict) -> list[dict]:
         if arr_path is None or not arr_path.is_file():
             continue
         try:
-            data = json.loads(arr_path.read_text(encoding="utf-8"))
+            data = load_json(arr_path)
         except (OSError, ValueError):
             continue
         if isinstance(data, dict):
@@ -219,7 +221,7 @@ def lift_sloppak(pak: Path, *, dry_run: bool = False) -> list[str]:
             continue
 
         try:
-            arr_data = json.loads(arr_path.read_text(encoding="utf-8"))
+            arr_data = load_json(arr_path)
         except (OSError, ValueError) as e:
             log.warning("%s/%s: unreadable arrangement JSON (%s) — skipped",
                         pak.name, arr_id, e)