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.test.ts

@@ -391,6 +391,21 @@ describe("UrlParams", () => {
     });
   });
 
+  describe("autoGainControl", () => {
+    it("defaults to true", () => {
+      expect(computeUrlParams().autoGainControl).toBe(true);
+    });
+
+    it("is parsed", () => {
+      expect(computeUrlParams("?autoGainControl=true").autoGainControl).toBe(
+        true,
+      );
+      expect(computeUrlParams("?autoGainControl=false").autoGainControl).toBe(
+        false,
+      );
+    });
+  });
+
   describe("header", () => {
     it("uses header if provided", () => {
       expect(computeUrlParams("?header=app_bar&hideHeader=true").header).toBe(