add \$B memory command

Registers 'memory' in META_COMMANDS, wires the meta-command dispatch
to a lazy-imported handler in memory-command.ts. Lazy because the
import graph (cdp-bridge + memory-snapshot + buffer accessors) isn't
useful to projects that never run the diagnostic.

The handler assembles MemoryStructureStats from the modules that own
each buffer (cdp-inspector mod history stats, activity subscriber
count, console/network/dialog buffer lengths, captureBuffer bytes,
inspectorSubscriber count via a new server.ts export) and calls
BrowserManager.getMemorySnapshot. Output is text by default, JSON with
--json so the sidebar footer and test harness can consume it
programmatically. buildMemorySnapshotJson is the entry the /memory
endpoint will call in the next commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

browse/src/server.ts

@@ -724,6 +724,11 @@ let inspectorTimestamp: number = 0;
 type InspectorSubscriber = (event: any) => void;
 const inspectorSubscribers = new Set<InspectorSubscriber>();
 
+/** Diagnostic accessor used by the $B memory snapshot. */
+export function getInspectorSubscriberCount(): number {
+  return inspectorSubscribers.size;
+}
+
 function emitInspectorEvent(event: any): void {
   for (const notify of inspectorSubscribers) {
     queueMicrotask(() => {