refactor: extract TabSession for per-tab state isolation (v0.15.16.0) (#873)

* plan: batch command endpoint + multi-tab parallel execution for GStack Browser

* refactor: extract TabSession from BrowserManager for per-tab state

Move per-tab state (refMap, lastSnapshot, frame) into a new TabSession
class. BrowserManager delegates to the active TabSession via
getActiveSession(). Zero behavior change — all existing tests pass.

This is the foundation for the /batch endpoint: both /command and /batch
will use the same handler functions with TabSession, eliminating shared
state races during parallel tab execution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: update handler signatures to use TabSession

Change handleReadCommand and handleSnapshot to take TabSession instead of
BrowserManager. Change handleWriteCommand to take both TabSession (per-tab
ops) and BrowserManager (global ops like viewport, headers, dialog).
handleMetaCommand keeps BrowserManager for tab management.

Tests use thin wrapper functions that bridge the old 3-arg call pattern to
the new signatures via bm.getActiveSession().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add POST /batch endpoint for parallel multi-tab execution

Execute multiple commands across tabs in a single HTTP request.
Commands targeting different tabs run concurrently via Promise.allSettled.
Commands targeting the same tab run sequentially within that group.

Features:
- Batch-safe command subset (text, goto, click, snapshot, screenshot, etc.)
- newtab/closetab as special commands within batch
- SSE streaming mode (stream: true) for partial results
- Per-command error isolation (one tab failing doesn't abort the batch)
- Max 50 commands per batch, soft batch-level timeout

A 143-page crawl drops from ~45 min (serial HTTP) to ~5 min (20 tabs
in parallel, batched commands).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: add batch endpoint integration tests

10 tests covering:
- Multi-tab parallel execution (goto + text on different tabs)
- Same-tab sequential ordering
- Per-command error isolation (one tab fails, others succeed)
- Page-scoped refs (snapshot refs are per-session, not global)
- Per-tab lastSnapshot (snapshot -D with independent baselines)
- getSession/getActiveSession API
- Batch-safe command subset validation
- closeTab via page.close preserves at-least-one-page invariant
- Parallel goto on 3 tabs simultaneously

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: harden codex-review E2E — extract SKILL.md section, bump maxTurns to 25

The test was copying the full 55KB/1075-line codex SKILL.md into the fixture,
requiring 8 Read calls just to consume it and exhausting the 15-turn budget
before reaching the actual codex review command. Now extracts only the
review-relevant section (~6KB/148 lines), reducing Read calls from 8 to 1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: move batch endpoint plan into BROWSER.md as feature documentation

The batch endpoint is implemented — document it as an actual feature in
BROWSER.md (architecture, API shape, design decisions, usage pattern)
and remove the standalone plan file.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: bump version and changelog (v0.15.16.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: gstack <ship@gstack.dev>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/src/server.ts

@@ -981,26 +981,28 @@ async function handleCommandInternal(
   try {
     let result: string;
 
+    const session = browserManager.getActiveSession();
+
     if (READ_COMMANDS.has(command)) {
       const isScoped = tokenInfo && tokenInfo.clientId !== 'root';
       // Hidden element stripping for scoped tokens on text command
       if (isScoped && command === 'text') {
-        const page = browserManager.getPage();
+        const page = session.getPage();
         const strippedDescs = await markHiddenElements(page);
         if (strippedDescs.length > 0) {
           console.warn(`[browse] Content security: stripped ${strippedDescs.length} hidden elements for ${tokenInfo.clientId}`);
         }
         try {
-          const target = browserManager.getActiveFrameOrPage();
+          const target = session.getActiveFrameOrPage();
           result = await getCleanTextWithStripping(target);
         } finally {
           await cleanupHiddenMarkers(page);
         }
       } else {
-        result = await handleReadCommand(command, args, browserManager);
+        result = await handleReadCommand(command, args, session);
       }
     } else if (WRITE_COMMANDS.has(command)) {
-      result = await handleWriteCommand(command, args, browserManager);
+      result = await handleWriteCommand(command, args, session, browserManager);
     } else if (META_COMMANDS.has(command)) {
       // Pass chain depth + executeCommand callback so chain routes subcommands
       // through the full security pipeline (scope, domain, tab, wrapping).
@@ -1021,7 +1023,7 @@ async function handleCommandInternal(
             return;
           }
           try {
-            const snapshot = await handleSnapshot(['-i'], browserManager);
+            const snapshot = await handleSnapshot(['-i'], browserManager.getActiveSession());
             browserManager.addWatchSnapshot(snapshot);
           } catch {
             // Page may be navigating — skip this snapshot