feat(core): consume song_timeline tempos + time_signatures + per-chart tempos (feedpak 1.2.0) (#529)

feedpak 1.2.0 added song-level `tempos` + `time_signatures` to song_timeline.json
and a per-chart `tempos` override on arrangements (§6.10). Core stored the raw
song_timeline dict but never consumed the maps, and didn't read per-chart tempos.

- song.py: shared `sanitize_tempos([{time,bpm}])` (finite non-bool time, finite
  bpm>0, sorted); `Arrangement.tempos` field wired through arrangement_to_wire
  (omitted when None/empty per §6.10) / arrangement_from_wire.
- sloppak.py: `_sanitize_time_signatures([{time,ts:[num,den]}])`;
  LoadedSloppak.tempos / .time_signatures, loaded from song_timeline.json
  INDEPENDENTLY of beats/sections (all are optional in 1.2.0).
- server.py: stream `tempos` + `time_signatures` highway-WS messages; the active
  arrangement's per-chart `tempos` overrides the song-level map for that chart.

Renderer/UI surfacing is a thin follow-up; this lands the data plumbing.

Codex-reviewed: clean (no findings). +9 tests (sanitizers, per-chart wire
round-trip + omit-when-absent, song-level load/sanitize/absent + maps-without-
beats). 90 song/sloppak tests pass. (Pre-existing unrelated failure:
test_diagnostics_redact, fails on clean main too.)

Closes #526. Part of got-feedback/feedback#334.

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

tests/test_sloppak_song_timeline_load.py

@@ -148,6 +148,50 @@ def test_song_timeline_absent_when_path_escapes_sloppak(tmp_path: Path):
     assert loaded.song_timeline is None
 
 
+# ── tempos + time_signatures (feedpak 1.2.0) ─────────────────────────────────
+
+def test_song_timeline_tempos_and_time_signatures_loaded(tmp_path: Path):
+    payload = {
+        "version": 1, "beats": [], "sections": [],
+        "tempos": [{"time": 0.0, "bpm": 120}, {"time": 4.0, "bpm": 90}],
+        "time_signatures": [{"time": 0.0, "ts": [4, 4]}, {"time": 8.0, "ts": [6, 8]}],
+    }
+    pak = _write_dir_sloppak(tmp_path, {"song_timeline": "song_timeline.json"}, payload)
+    loaded = _load(pak, tmp_path)
+    assert loaded.tempos == [{"time": 0.0, "bpm": 120.0}, {"time": 4.0, "bpm": 90.0}]
+    assert loaded.time_signatures == [{"time": 0.0, "ts": [4, 4]},
+                                      {"time": 8.0, "ts": [6, 8]}]
+
+
+def test_song_timeline_maps_absent_when_not_provided(tmp_path: Path):
+    payload = {"version": 1, "beats": [], "sections": []}
+    pak = _write_dir_sloppak(tmp_path, {"song_timeline": "song_timeline.json"}, payload)
+    loaded = _load(pak, tmp_path)
+    assert loaded.tempos is None and loaded.time_signatures is None
+
+
+def test_song_timeline_maps_sanitized(tmp_path: Path):
+    payload = {
+        "version": 1, "beats": [], "sections": [],
+        "tempos": [{"time": 1.0, "bpm": 0}, {"time": 0.0, "bpm": 100}],   # bpm 0 dropped + sorted
+        "time_signatures": [{"time": 0.0, "ts": [4, 4, 4]},                # 3-long dropped
+                            {"time": 2.0, "ts": [3, 4]}],
+    }
+    pak = _write_dir_sloppak(tmp_path, {"song_timeline": "song_timeline.json"}, payload)
+    loaded = _load(pak, tmp_path)
+    assert loaded.tempos == [{"time": 0.0, "bpm": 100.0}]
+    assert loaded.time_signatures == [{"time": 2.0, "ts": [3, 4]}]
+
+
+def test_song_timeline_maps_load_even_without_beats_or_sections(tmp_path: Path):
+    # tempos/time_signatures are independent of beats/sections — a payload that
+    # omits beats (invalid for the override path) must still surface the maps.
+    payload = {"version": 1, "tempos": [{"time": 0.0, "bpm": 100}]}
+    pak = _write_dir_sloppak(tmp_path, {"song_timeline": "song_timeline.json"}, payload)
+    loaded = _load(pak, tmp_path)
+    assert loaded.tempos == [{"time": 0.0, "bpm": 100.0}]
+
+
 def test_song_timeline_absent_when_path_is_absolute(tmp_path: Path):
     pak = _write_dir_sloppak(tmp_path, {"song_timeline": "/etc/passwd"}, None)
     loaded = _load(pak, tmp_path)