feat(lotus-denoise): AGC off for ML tier + init/build leak hardening

AEC/AGC audit fix + two hardening items from the engine review.

- Add an `autoGainControl` capture param (UrlParams -> CallViewModel ->
  ConnectionFactory audioCaptureDefaults), mirroring echoCancellation/
  noiseSuppression. Defaults true (unchanged); the host sets it false only for
  the ML tier so the browser's auto gain control doesn't fight the in-source ML
  denoiser (pumping). Echo cancellation stays on. Tests cover the URL parse and
  the audioCaptureDefaults wiring.
- L1: init() now closes the owned AudioContext on a build failure (was orphaned;
  browsers cap live contexts, so repeated failures could exhaust them).
- L2: buildGraph() disposes its partially-built nodes on failure (disposeGraph
  previously only cleaned the prior graph).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/state/CallViewModel/CallViewModel.ts

@@ -504,6 +504,7 @@ export function createCallViewModel$(
       options.livekitRoomFactory,
       getUrlParams().echoCancellation,
       getUrlParams().noiseSuppression,
+      getUrlParams().autoGainControl,
     );
 
   const connectionManager = createConnectionManager$({