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/read-commands.ts

@@ -5,7 +5,7 @@
  * console, network, cookies, storage, perf
  */
 
-import type { BrowserManager } from './browser-manager';
+import type { TabSession } from './tab-session';
 import { consoleBuffer, networkBuffer, dialogBuffer } from './buffers';
 import type { Page, Frame } from 'playwright';
 import * as fs from 'fs';
@@ -94,11 +94,11 @@ export async function getCleanText(page: Page | Frame): Promise<string> {
 export async function handleReadCommand(
   command: string,
   args: string[],
-  bm: BrowserManager
+  session: TabSession
 ): Promise<string> {
-  const page = bm.getPage();
+  const page = session.getPage();
   // Frame-aware target for content extraction
-  const target = bm.getActiveFrameOrPage();
+  const target = session.getActiveFrameOrPage();
 
   switch (command) {
     case 'text': {
@@ -108,7 +108,7 @@ export async function handleReadCommand(
     case 'html': {
       const selector = args[0];
       if (selector) {
-        const resolved = await bm.resolveRef(selector);
+        const resolved = await session.resolveRef(selector);
         if ('locator' in resolved) {
           return await resolved.locator.innerHTML({ timeout: 5000 });
         }
@@ -190,7 +190,7 @@ export async function handleReadCommand(
     case 'css': {
       const [selector, property] = args;
       if (!selector || !property) throw new Error('Usage: browse css <selector> <property>');
-      const resolved = await bm.resolveRef(selector);
+      const resolved = await session.resolveRef(selector);
       if ('locator' in resolved) {
         const value = await resolved.locator.evaluate(
           (el, prop) => getComputedStyle(el).getPropertyValue(prop),
@@ -212,7 +212,7 @@ export async function handleReadCommand(
     case 'attrs': {
       const selector = args[0];
       if (!selector) throw new Error('Usage: browse attrs <selector>');
-      const resolved = await bm.resolveRef(selector);
+      const resolved = await session.resolveRef(selector);
       if ('locator' in resolved) {
         const attrs = await resolved.locator.evaluate((el) => {
           const result: Record<string, string> = {};
@@ -276,7 +276,7 @@ export async function handleReadCommand(
       const selector = args[1];
       if (!property || !selector) throw new Error('Usage: browse is <property> <selector>\nProperties: visible, hidden, enabled, disabled, checked, editable, focused');
 
-      const resolved = await bm.resolveRef(selector);
+      const resolved = await session.resolveRef(selector);
       let locator;
       if ('locator' in resolved) {
         locator = resolved.locator;