fix(rtmg/web): re-send the active loop band on reconnect

[seanhanca]

Root cause (confirmed)

After an automatic reconnect (network blip), the active loop band is silently dropped server-side.

The WsReconnector success callback in demos/realtime_motion_graph_web/web/hooks/useStartSession.ts re-applies detected metadata, the LoRA catalog/cap/strengths, the network monitor, and timbre/structure refs (await restoreRefs(remote)), but never re-sends the loop band:

• The loop band is not part of buildConfig (useStartSession.ts buildConfig, ~L100-136) — it carries prompts, LoRAs, fixture, source mode, client id, but no band.
• The only code that calls remote.sendLoopBand(...) is the WaveformScrubBox sync effect (components/Performance/WaveformScrubBox.tsx ~L361-384), which depends on [bandState, bandLoopEnabled, hasPlayer, player]. On reconnect the AudioPlayer persists and loopBand/bandLoopEnabled are unchanged, so the effect does not re-run.

Result: the new RemoteBackend never learns the band. The worklet keeps looping the region locally while the new server session decodes against the full timeline → drift / a stale loop window at the seam.

sendLoopBand is a real SDK method (packages/demon-client/protocol.ts ~L1070), so the fix is wire-supported.

Fix

Mirror the established "re-apply session state on reconnect" pattern (restoreRefs):

• Add restoreLoopBand(remote) next to restoreRefs. It reads the source-of-truth performance store (usePerformanceStore loopBand / bandLoopEnabled) and, when a band is active, re-sends it via remote.sendLoopBand(start, end).
• The activity gate matches WaveformScrubBox exactly (band != null && bandLoopEnabled && end-start >= 0.05 && start >= 0) so we only re-send a band the listener is actually hearing loop.
• Call it right after await restoreRefs(remote) in the WsReconnector success callback.

No new subsystem; ~30 lines including doc comment, following the same extracted-function shape as restoreRefs.

No regression

Extended tests/unit/wsReconnect.test.ts with a restoreLoopBand on reconnect block asserting:

• the active loop band is re-sent with its exact region after a reconnect;
• nothing is sent when no band is set;
• nothing is sent for an armed-but-off (loop-disabled) band;
• a degenerate sub-50ms region is ignored.

Verification

Run in demos/realtime_motion_graph_web/web/:

• npx vitest run tests/unit/wsReconnect.test.ts → 11 passed (7 existing + 4 new).
• npm run typecheck → clean (exit 0).
• npm run build → compiled successfully (exit 0).

(The full tests/unit run shows 8 pre-existing failures in sliceEpoch.test.ts due to Math.f16round is not a function on this Node build — unrelated to this change; the touched test file passes.)

Made with Cursor