feat(audio): getLatencyBreakdown — one owner for every latency term

Deep-read §5: latency had three unreconciled truths — getLatencyMs' static
half-capacity ring guess (42.7 ms), the verifier's input-delta-only offset,
and the renderer bus adding prime+fill+resample that no figure surfaced.

New engine API + export: per-term breakdown (deviceBufferMs, input/output
driver latency, MEASURED split-ring residency, monitor total) plus the
renderer-bus song-audio delay (measured bus fill) as its own term. On the
user's split exclusive setup the measured ring sits at ~10 ms — the legacy
figure overstated monitor latency by ~33 ms (102.7 reported vs ~70 real).

getLatencyMs is unchanged for compatibility; UI adoption of the breakdown
is renderer follow-up. Snapshots regenerated (104 exports).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/contracts/addon-exports.json

@@ -17,6 +17,7 @@
   "getDeviceMetrics",
   "getDeviceTypes",
   "getKnownPlugins",
+  "getLatencyBreakdown",
   "getLevels",
   "getMonitorMuteState",
   "getNoteVerdicts",