refactor(extension): class-based command registry + modular src layout

Restructure the MV3 background worker from a monolithic core.ts/index.ts
into a class-based command architecture. Behavior is identical — the 83
registered commands dispatch byte-for-byte the same as before.

Structure
- One class per command group, each extending CommandGroup and exporting a
  `commands` map keyed by the full command id ("tabs.close"). Groups:
  Navigation, TabsMutation, TabsQuery, Groups, Windows, Dom (dom/extract/
  page), BrowserData (storage/cookies), Session (session/clients + autosave
  + lazy-tab activation), Perf (perf + jobs.status/cancel), Extension.
- CommandRegistry merges the group maps (throws on duplicate ids), routes
  background specs to JobManager and paginates array results via
  makePagedData. JobManager owns the job map + lifecycle. NativeConnection
  owns the native-port lifecycle and the inbound message router.
- index.ts is now thin wiring: JobManager -> ctx -> assembleRegistry ->
  onActivated -> NativeConnection.start().
- Infra classes live in classes/ (PascalCase, file = class name); command
  groups in commands/; shared helpers split out of core.ts into core/
  (errors, throttle, scripting, tab-helpers, group-helpers, storage); all
  types moved into types/ (json, jobs, session, tabs, messages,
  command-args) behind a barrel.

DRY cleanup
- resolveTabUrl(tabId) and assertScriptableUrl(url, action) collapse the
  tab/URL-guard boilerplate duplicated across dom.ts and browser-data.ts.
- processInBatches() centralizes the throttled, cancellable batch loop
  shared by tabs.close, group.close and tabs.merge_windows.
- captureCurrentSession() dedups the snapshot-and-signature block shared by
  session.save and the autosave path.
- DomArgs type alias replaces 21 inline ContentArgs & { tabId? } copies.
- Drop fetchTabHtml's redundant retry loop (executeScript already retries
  transient frame/tab errors), a dead tabInfo import, and two stale
  comments referencing a removed asArgs helper.

Type safety & tests
- Full noImplicitAny; no `any`/`unknown` annotations remain in src.
- JS unit-test harness using node --test + node:assert (zero new deps),
  bundled via the existing esbuild. Covers JobManager retention/lifecycle
  and the autosave listener-wiring/debounce with an in-memory chrome mock.
- The structural pytest checks track the new file homes and the centralized
  processInBatches helper.

Verification: npm run check:extension green (tsc + esbuild 84.5kb +
node --check + 18 JS tests); uv run pytest -q -> 409 passed, 105 skipped.
No version bump.

extension/src/core/throttle.ts

@@ -0,0 +1,112 @@
+// Large-operation throttling, performance profile, and job-progress helpers.
+import type { Job, JobProgressUpdate } from '../types';
+
+export function sleep(ms: number): Promise<void> {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
+export const LARGE_OPERATION_BATCH_SIZE = 25;
+export const LARGE_OPERATION_PAUSE_MS = 25;
+export const GENTLE_OPERATION_BATCH_SIZE = 8;
+export const GENTLE_OPERATION_PAUSE_MS = 100;
+
+export async function hasAudibleTabs() {
+  const audibleTabs = await chrome.tabs.query({ audible: true });
+  return audibleTabs.some(tab => !(tab.mutedInfo && tab.mutedInfo.muted));
+}
+
+let largeOperationQueue: Promise<void> = Promise.resolve();
+
+export async function runLargeOperation<T>(name: string, fn: () => Promise<T>): Promise<T> {
+  const run = largeOperationQueue.then(async () => {
+    console.log(`[browser-cli] large operation start: ${name}`);
+    try {
+      return await fn();
+    } finally {
+      console.log(`[browser-cli] large operation done: ${name}`);
+    }
+  });
+  largeOperationQueue = run.then(() => {}, () => {});
+  return run;
+}
+
+export async function getPerformanceProfile() {
+  const { performanceProfile } = await chrome.storage.local.get<{ performanceProfile?: string }>("performanceProfile");
+  return performanceProfile || "auto";
+}
+
+export async function setPerformanceProfile(profile: string) {
+  const allowed = new Set(["auto", "normal", "gentle", "ultra"]);
+  const performanceProfile = allowed.has(profile) ? profile : "auto";
+  await chrome.storage.local.set({ performanceProfile });
+  return { performanceProfile };
+}
+
+export async function getLargeOperationThrottle(itemCount = 0, mode: string = "auto") {
+  const audible = await hasAudibleTabs();
+  const storedProfile = await getPerformanceProfile();
+  const configuredMode = mode && mode !== "auto" ? mode : storedProfile;
+  const gentle = configuredMode === "gentle" || configuredMode === "ultra" || (configuredMode === "auto" && audible);
+  let batchSize = gentle ? GENTLE_OPERATION_BATCH_SIZE : LARGE_OPERATION_BATCH_SIZE;
+  let pauseMs = gentle ? GENTLE_OPERATION_PAUSE_MS : LARGE_OPERATION_PAUSE_MS;
+
+  if (configuredMode === "ultra" || itemCount >= 300) {
+    batchSize = Math.max(3, Math.floor(batchSize / 2));
+    pauseMs *= 2;
+  } else if (itemCount >= 100) {
+    batchSize = Math.max(5, Math.floor(batchSize * 0.75));
+    pauseMs = Math.max(pauseMs, 75);
+  }
+
+  return { batchSize, pauseMs, gentle, audible, itemCount, mode: configuredMode };
+}
+
+export function updateJobProgress(job: Job | undefined | null, { phase, current, total }: JobProgressUpdate = {}) {
+  if (!job) return;
+  if (phase) job.phase = phase;
+  if (total != null) job.total = total;
+  if (current != null) job.current = current;
+  if (job.total) job.percent = Math.min(100, Math.round((job.current || 0) * 100 / job.total));
+  job.updatedAt = Date.now();
+}
+
+export function throwIfJobCancelled(job: Job | undefined | null) {
+  if (job?.cancelRequested) {
+    throw new Error(`Job '${job.id}' cancelled`);
+  }
+}
+
+export async function yieldForLargeOperation(processed: number, batchSize = LARGE_OPERATION_BATCH_SIZE, pauseMs = LARGE_OPERATION_PAUSE_MS) {
+  if (processed > 0 && processed % batchSize === 0) {
+    await sleep(pauseMs);
+  }
+}
+
+/**
+ * Run `handler` over `items` in throttle-sized slices, threading job progress
+ * and cancellation. Each slice is one `handler` call (so chrome batch APIs like
+ * tabs.remove get an array); between slices it reports progress and yields.
+ *
+ * `total`/`baseCurrent` let callers report against a fixed total accumulated
+ * across several calls (e.g. merging tabs from many windows). Returns the
+ * running processed count so the next call can continue from it.
+ */
+export async function processInBatches<T>(
+  items: T[],
+  throttle: { batchSize: number; pauseMs: number },
+  handler: (batch: T[]) => Promise<unknown>,
+  progress: { job?: Job | null; phase: string; total?: number; baseCurrent?: number },
+): Promise<number> {
+  const total = progress.total ?? items.length;
+  let done = progress.baseCurrent ?? 0;
+  updateJobProgress(progress.job, { phase: progress.phase, current: done, total });
+  for (let i = 0; i < items.length; i += throttle.batchSize) {
+    throwIfJobCancelled(progress.job);
+    const batch = items.slice(i, i + throttle.batchSize);
+    await handler(batch);
+    done += batch.length;
+    updateJobProgress(progress.job, { phase: progress.phase, current: done, total });
+    await yieldForLargeOperation(done, throttle.batchSize, throttle.pauseMs);
+  }
+  return done;
+}