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/UrlParams.ts

@@ -239,6 +239,12 @@ export interface UrlConfiguration {
    * Defaults to true.
    */
   noiseSuppression?: boolean;
+  /**
+   * Whether to enable auto gain control for audio capture.
+   * Defaults to true. Lotus turns this OFF for the in-source ML denoise tier so
+   * the browser's dynamic gain doesn't fight the ML model (pumping artifacts).
+   */
+  autoGainControl?: boolean;
 
   callIntent?: RTCCallIntent;
 }
@@ -485,6 +491,7 @@ export const computeUrlParams = (search = "", hash = ""): UrlParams => {
     autoLeaveWhenOthersLeft: parser.getFlag("autoLeave"),
     noiseSuppression: parser.getFlagParam("noiseSuppression", true),
     echoCancellation: parser.getFlagParam("echoCancellation", true),
+    autoGainControl: parser.getFlagParam("autoGainControl", true),
   };
 
   // Log the final configuration for debugging purposes.