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.

package.json

@@ -4,7 +4,9 @@
   "type": "module",
   "scripts": {
     "build:extension": "esbuild extension/src/index.ts --bundle --format=iife --target=chrome120 --outfile=extension/background.js",
-    "check:extension": "tsc -p tsconfig.json --noEmit && npm run build:extension && node --check extension/background.js"
+    "build:tests": "esbuild extension/test/*.test.ts --bundle --format=esm --platform=node --outdir=extension/test-dist --out-extension:.js=.mjs",
+    "test:extension": "npm run build:tests && node --test extension/test-dist/*.test.mjs",
+    "check:extension": "tsc -p tsconfig.json --noEmit && npm run build:extension && node --check extension/background.js && npm run test:extension"
   },
   "devDependencies": {
     "@types/chrome": "^0.1.40",