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>

lib/song.py

@@ -151,6 +151,10 @@ class Arrangement:
     # RS2014 custom song pitch-shift field (cents). Commonly -1200.0 (one octave
     # down) for extended-range bass arrangements. 0.0 when absent or zero.
     cent_offset: float = 0.0
+    # Per-chart tempo override (§6.10): [{time, bpm}]. None when the chart
+    # follows the song-level tempo; when present a Reader uses it for this
+    # chart and ignores the song-level tempo.
+    tempos: list | None = None
 
 
 @dataclass
@@ -585,6 +589,29 @@ def _finite_float(value, default: float = 0.0) -> float:
     return v if math.isfinite(v) else default
 
 
+def sanitize_tempos(events) -> list[dict]:
+    """Clean a tempo-event list (``[{time, bpm}]``): keep entries with a finite
+    non-bool ``time`` and a finite ``bpm > 0``, coerced to float and sorted by
+    time. Non-list / all-invalid input -> ``[]``. Shared by the per-chart
+    arrangement ``tempos`` (§6.10) and the song-level ``song_timeline.tempos``."""
+    out: list[dict] = []
+    if isinstance(events, list):
+        for ev in events:
+            if not isinstance(ev, dict):
+                continue
+            t = ev.get("time")
+            bpm = ev.get("bpm")
+            if (not isinstance(t, (int, float)) or isinstance(t, bool)
+                    or not math.isfinite(t)):
+                continue
+            if (not isinstance(bpm, (int, float)) or isinstance(bpm, bool)
+                    or not math.isfinite(bpm) or bpm <= 0):
+                continue
+            out.append({"time": float(t), "bpm": float(bpm)})
+        out.sort(key=lambda e: e["time"])
+    return out
+
+
 def arrangement_to_wire(arr: Arrangement) -> dict:
     """Serialize an Arrangement into a JSON-ready dict matching the wire format."""
     out = {
@@ -612,6 +639,10 @@ def arrangement_to_wire(arr: Arrangement) -> dict:
     # "no tones".
     if arr.tones:
         out["tones"] = arr.tones
+    # Per-chart tempo override (§6.10) — additive; omit when the chart follows
+    # the song-level tempo (empty/None).
+    if arr.tempos:
+        out["tempos"] = list(arr.tempos)
     return out
 
 
@@ -622,6 +653,7 @@ def arrangement_from_wire(d: dict) -> Arrangement:
         tuning=list(d.get("tuning", [0] * 6)),
         capo=int(d.get("capo", 0)),
         cent_offset=_finite_float(d.get("centOffset", 0.0)),
+        tempos=(sanitize_tempos(d.get("tempos")) or None),
         notes=[note_from_wire(n) for n in d.get("notes", [])],
         chords=[chord_from_wire(c) for c in d.get("chords", [])],
         anchors=[