add BrowserManager.getMemorySnapshot() + shared types

Diagnostic foundation for $B memory and the /memory endpoint that land
in the next two commits. Collects:

- Bun process memory via process.memoryUsage (cross-platform, accurate).
- Per-tab JS heap via CDP Performance.getMetrics, lazy per tracked page,
  swallows target-died errors so a dying tab doesn't poison the
  snapshot for the rest.
- Chromium process tree via SystemInfo.getProcessInfo (PID + type +
  CPU time). RSS is NOT exposed via CDP — the eng review (D2 USE_CDP)
  picked CDP over shelling to `ps`, so notes[] tells the caller why
  the RSS column is absent and points at the follow-up TODO.

cdp-inspector exports getModificationHistoryStats so the snapshot can
surface buffer occupancy + cap + evicted count without reaching into
module-private state.

memory-snapshot.ts holds the shared types so server.ts and read-commands
can import without circular dep on browser-manager.

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

browse/src/cdp-inspector.ts

@@ -670,6 +670,23 @@ export function getModificationHistory(): StyleModification[] {
   return [...modificationHistory];
 }
 
+/**
+ * Diagnostic accessor for the $B memory snapshot. Returns current buffer
+ * occupancy, the cap, and how many entries have been evicted since the
+ * last reset.
+ */
+export function getModificationHistoryStats(): {
+  current: number;
+  cap: number;
+  evicted: number;
+} {
+  return {
+    current: modificationHistory.length,
+    cap: MOD_HISTORY_CAP,
+    evicted: Math.max(0, modHistoryTotalPushed - MOD_HISTORY_CAP),
+  };
+}
+
 /**
  * Reset all modifications, restoring original values.
  */