fix: install bounded Browser RPC stream leases

R-04: install the RPC contract bindings as exact immutable snapshots. Registry
and row data are copied from own data descriptors into frozen null-prototype
maps before validation, so a getter is never invoked, extra and symbol keys and
malformed descriptors are composition-time TypeErrors, and the runtime reads
only the snapshot. A post-validation mutation can no longer change replay
policy, deadlines, byte ceilings or transport selection.

R-01: bound transport stream cleanup. The generation is fenced and listeners
released immediately, and iterator.return() is awaited only within a cleanup
bound, so a non-cooperative iterator cannot keep the application generator, its
listeners or the total deadline alive. Unresolved cleanup stays observed.

R-05: reject oversized WebSocket text frames before allocating an encoded copy
and count UTF-8 bytes incrementally with an early exit, matching TextEncoder for
surrogate pairs and lone surrogates.

R-06: canonicalise clock and generation-fence failures into the closed Result
taxonomy instead of letting them escape as native rejections, with listener and
timer cleanup on every exit path.

Browser RPC remains AVAILABLE_NOT_COMPOSED; R-07 transport evidence is still
required before composition.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/browser-rpc/browser-rpc-contract.test.ts

@@ -6,6 +6,7 @@ import {
   composeBrowserRpcRequestEncoderRegistry,
   defineBrowserRpcOperation,
   defineBrowserRpcProviderProfile,
+  installBrowserRpcContractBindings,
   validateBrowserRpcContractBindings,
   type BrowserRpcProviderProfile,
 } from "../../../src/contracts/browser-rpc.ts";
@@ -23,6 +24,91 @@ import {
 } from "./fixture.ts";
 
 describe("Browser RPC contract registry", () => {
+  it("snapshots installed bindings before later source mutation", () => {
+    const operations: Record<string, ReturnType<typeof unaryOperation>> = {
+      GET_RPC_RESOURCE: unaryOperation(),
+    };
+    const installed = installBrowserRpcContractBindings({
+      operations,
+      profiles: { [unaryProfile().runtimeProfileId]: unaryProfile() },
+      schemaCodecs: SCHEMA_CODECS,
+      mappers: MAPPERS,
+      requestEncoders: { RpcResourceRequestEncoder: UNARY_ENCODER },
+    });
+    const before = installed.operations.get("GET_RPC_RESOURCE");
+    expect(before?.totalDeadlineMs).toBeDefined();
+
+    // R-04. A post-validation mutation of the source registry must not reach
+    // the installed snapshot.
+    operations.GET_RPC_RESOURCE = {
+      ...operations.GET_RPC_RESOURCE!,
+      totalDeadlineMs: 999_999,
+    };
+    expect(installed.operations.get("GET_RPC_RESOURCE")).toBe(before);
+    expect(
+      installed.operations.get("GET_RPC_RESOURCE")?.totalDeadlineMs,
+    ).not.toBe(999_999);
+  });
+
+  it("rejects extra accessor and symbol keys without invoking getters", () => {
+    let getterCalls = 0;
+    const accessorOperation = Object.defineProperty(
+      { ...unaryOperation() },
+      "totalDeadlineMs",
+      {
+        enumerable: true,
+        configurable: true,
+        get() {
+          getterCalls += 1;
+          return 1_000;
+        },
+      },
+    );
+    expect(() =>
+      installBrowserRpcContractBindings({
+        operations: { GET_RPC_RESOURCE: accessorOperation },
+        profiles: { [unaryProfile().runtimeProfileId]: unaryProfile() },
+        schemaCodecs: SCHEMA_CODECS,
+        mappers: MAPPERS,
+        requestEncoders: { RpcResourceRequestEncoder: UNARY_ENCODER },
+      }),
+    ).toThrow(TypeError);
+    expect(getterCalls).toBe(0);
+
+    const extraKeyOperation = {
+      ...unaryOperation(),
+      unexpectedKey: "smuggled",
+    };
+    expect(() =>
+      installBrowserRpcContractBindings({
+        operations: {
+          GET_RPC_RESOURCE: extraKeyOperation as never,
+        },
+        profiles: { [unaryProfile().runtimeProfileId]: unaryProfile() },
+        schemaCodecs: SCHEMA_CODECS,
+        mappers: MAPPERS,
+        requestEncoders: { RpcResourceRequestEncoder: UNARY_ENCODER },
+      }),
+    ).toThrow(/unexpected key/u);
+
+    const symbolRegistry: Record<string, unknown> = {
+      GET_RPC_RESOURCE: unaryOperation(),
+    };
+    Object.defineProperty(symbolRegistry, Symbol("hidden"), {
+      enumerable: true,
+      value: unaryOperation(),
+    });
+    expect(() =>
+      installBrowserRpcContractBindings({
+        operations: symbolRegistry as never,
+        profiles: { [unaryProfile().runtimeProfileId]: unaryProfile() },
+        schemaCodecs: SCHEMA_CODECS,
+        mappers: MAPPERS,
+        requestEncoders: { RpcResourceRequestEncoder: UNARY_ENCODER },
+      }),
+    ).toThrow(/symbol keys/u);
+  });
+
   it("closes exact operation, provider, schema, mapper and encoder bindings", () => {
     const operation = unaryOperation();
     const profile = unaryProfile();