fix(lotus): set_quality null actually clears a cap on the sender

A null cap only removed the key from the sticky settings and applyToRoom
then skipped that sender, leaving the previously written maxBitrate /
maxFramerate live on the RTCRtpSender — contrary to the host contract
documented in cinny's CallControl. Track which keys this module wrote per
sender and write them back as undefined on clear. Unit-tested.

Fixes #11

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/lotus/lotusQuality.test.ts

@@ -0,0 +1,93 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, test, vi } from "vitest";
+
+import { buildPatch, patchSender } from "./lotusQuality";
+
+function makeSender(
+  initialEncodings: RTCRtpEncodingParameters[] = [{}],
+): RTCRtpSender {
+  let encodings = initialEncodings;
+  return {
+    getParameters: vi.fn(() => ({ encodings })),
+    setParameters: vi.fn(async (params: RTCRtpSendParameters) => {
+      await Promise.resolve();
+      encodings = params.encodings ?? [];
+    }),
+  } as unknown as RTCRtpSender;
+}
+
+describe("lotusQuality set_quality -> clear (#11)", () => {
+  test("clearing a previously-set cap actively unsets it on the sender", async () => {
+    const sender = makeSender();
+    const writtenKeys = new WeakMap<
+      RTCRtpSender,
+      Set<keyof RTCRtpEncodingParameters>
+    >();
+
+    // Set: audioMaxBitrate = 64000.
+    const setPatch = buildPatch(sender, { maxBitrate: 64_000 }, writtenKeys);
+    expect(setPatch).toEqual({ maxBitrate: 64_000 });
+    await patchSender(sender, setPatch, writtenKeys);
+
+    expect(sender.getParameters().encodings[0].maxBitrate).toBe(64_000);
+    expect(writtenKeys.get(sender)).toEqual(new Set(["maxBitrate"]));
+
+    // Clear: host sends `null`, so the caller now wants an empty desired
+    // patch. buildPatch must still emit an explicit `undefined` for the key
+    // it previously wrote, instead of an empty patch that leaves the stale
+    // cap on the sender.
+    const clearPatch = buildPatch(sender, {}, writtenKeys);
+    expect(clearPatch).toEqual({ maxBitrate: undefined });
+    await patchSender(sender, clearPatch, writtenKeys);
+
+    const finalEncoding = sender.getParameters().encodings[0];
+    expect(finalEncoding.maxBitrate).toBeUndefined();
+    expect("maxBitrate" in finalEncoding).toBe(true); // explicitly cleared, not merely absent
+    expect(writtenKeys.has(sender)).toBe(false);
+  });
+
+  test("clearing one of several caps leaves the others (and their tracking) intact", async () => {
+    const sender = makeSender();
+    const writtenKeys = new WeakMap<
+      RTCRtpSender,
+      Set<keyof RTCRtpEncodingParameters>
+    >();
+
+    const setPatch = buildPatch(
+      sender,
+      { maxBitrate: 500_000, maxFramerate: 24 },
+      writtenKeys,
+    );
+    await patchSender(sender, setPatch, writtenKeys);
+    expect(writtenKeys.get(sender)).toEqual(
+      new Set(["maxBitrate", "maxFramerate"]),
+    );
+
+    // Only maxFramerate is still desired; maxBitrate should be actively
+    // cleared.
+    const clearPatch = buildPatch(sender, { maxFramerate: 24 }, writtenKeys);
+    expect(clearPatch).toEqual({ maxFramerate: 24, maxBitrate: undefined });
+    await patchSender(sender, clearPatch, writtenKeys);
+
+    const finalEncoding = sender.getParameters().encodings[0];
+    expect(finalEncoding.maxBitrate).toBeUndefined();
+    expect(finalEncoding.maxFramerate).toBe(24);
+    expect(writtenKeys.get(sender)).toEqual(new Set(["maxFramerate"]));
+  });
+
+  test("no sender means an empty patch and no call", () => {
+    const writtenKeys = new WeakMap<
+      RTCRtpSender,
+      Set<keyof RTCRtpEncodingParameters>
+    >();
+    expect(buildPatch(undefined, { maxBitrate: 1000 }, writtenKeys)).toEqual(
+      {},
+    );
+  });
+});