fix: keep Browser RPC collaborator input and output inside the contract

RPC-RR-02. The server-stream path captured the generation fence outside its
protected boundary and raceWithin invoked clock.sleep outside a promise
boundary, so a synchronous throw from either escaped the Result contract and
skipped the listener and timer release. Both now run inside the boundary, and
release moved to finally.

RPC-RR-03. The runtime snapshotted its transports only after validating the
caller's raw objects, which ran their accessors first. It now decodes the
registry from own data descriptors before anything reads it — refusing an
accessor without invoking it and rejecting extra, inherited and symbol-keyed
fields — and validates that snapshot. Every installed binding registry is a
read facade over a private store instead of a frozen Map whose set, delete and
clear still worked.

RPC-RR-04. Transport results and stream frames are decoded per union variant
from own data descriptors into new frozen values. A throwing getter, an
inherited or extra field, a symbol key, an unknown failure code and an
out-of-range retryAfterMs all close as protocol failures instead of escaping.

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

src/contracts/browser-rpc.ts

@@ -1,3 +1,7 @@
+import {
+  createReadOnlyRegistry,
+  type ReadOnlyRegistry,
+} from "./read-only-registry.ts";
 import type { InstalledBoundaryMapper } from "./boundary-mapper.ts";
 import type { RuntimeSchemaCodec } from "./schema-registry.ts";
 
@@ -327,13 +331,18 @@ export function composeBrowserRpcRequestEncoderRegistry(
   );
 }
 
+/**
+ * RPC-RR-03. Read facades, never `Map`s. `Object.freeze(new Map(...))` leaves
+ * `set`, `delete` and `clear` working, so an installed registry could still be
+ * emptied or re-pointed after the snapshot was validated.
+ */
 export type InstalledBrowserRpcContractBindings = Readonly<{
-  operations: ReadonlyMap<string, BrowserRpcOperationV3>;
-  profiles: ReadonlyMap<string, BrowserRpcProviderProfile>;
-  schemaCodecs: ReadonlyMap<string, RuntimeSchemaCodec>;
-  mappers: ReadonlyMap<string, InstalledBoundaryMapper>;
-  requestEncoders: ReadonlyMap<string, BrowserRpcRequestEncoder>;
-  runtimeBindings: ReadonlyMap<string, BrowserRpcRuntimeBindingIdentity>;
+  operations: ReadOnlyRegistry<string, BrowserRpcOperationV3>;
+  profiles: ReadOnlyRegistry<string, BrowserRpcProviderProfile>;
+  schemaCodecs: ReadOnlyRegistry<string, RuntimeSchemaCodec>;
+  mappers: ReadOnlyRegistry<string, InstalledBoundaryMapper>;
+  requestEncoders: ReadOnlyRegistry<string, BrowserRpcRequestEncoder>;
+  runtimeBindings: ReadOnlyRegistry<string, BrowserRpcRuntimeBindingIdentity>;
 }>;
 
 /**
@@ -351,7 +360,7 @@ function installRegistrySnapshot<Value extends object>(
   source: Readonly<Record<string, Value>>,
   label: string,
   allowedKeys: readonly string[],
-): ReadonlyMap<string, Value> {
+): ReadOnlyRegistry<string, Value> {
   let ownKeys: string[];
   let symbols: readonly symbol[];
   try {
@@ -376,7 +385,7 @@ function installRegistrySnapshot<Value extends object>(
       installRowSnapshot(descriptor.value as Value, `${label}.${key}`, allowedKeys),
     );
   }
-  return Object.freeze(installed) as ReadonlyMap<string, Value>;
+  return createReadOnlyRegistry(installed);
 }
 
 function installRowSnapshot<Value extends object>(