fix(browser): close the browser when its page cannot be opened

[mrgoonie]

Fixes #579. Refs #564 Depends on #566. This branch is stacked on fix/564-driver-close-during-launch; the diff against main includes #566's commits until it lands. Rebase/merge main once #566 merges. ## Problem In packs/browser-playwright/src/driver.ts (#launch, reached from #ensurePage), context.newPage() ran outside the try that closes a failed start. If it threw, the persistent context stayed open holding the profile folder, #context still pointed at it, and the next call launched a second Chromium on the same profile. ## Fix Opening the page (context.pages() / context.newPage()) now sits inside the existing cleanup try, so a failure resets #context and closes the browser it started. #launching is already cleared by the finally in #ensurePage, so the next call starts cleanly; the closed/launching state machine from #566 is unchanged. ## Test New packs/browser-playwright/test/new-page-failure.spec.ts launches real Chromium (same pattern as close-during-launch.spec.ts), makes the first start report no pages and a failing newPage, and checks: - the first observe() rejects and the browser it started is disconnected; - the next observe() succeeds on a second launch, with exactly one browser still connected. Against the unfixed driver it fails (launched[0].browser().isConnected() is true). ## Verification - vitest run packs/browser-playwright apps/runtime/test/{browser-effects,session-preview-real,task-browser}.spec.ts: 9 files, 71 tests passed - pnpm typecheck, pnpm invariants, eslint on changed files: pass - pnpm verify: invariants/typecheck/lint pass; the full test run was on a machine at 100% CPU with other sessions' Chromium processes, and two runs failed different, unrelated timing/EPERM specs (worktree-sweep, managed-worktree, scan-secret-history, package-fetch, scoped-fs, and once the browser pack's 20s timeouts). Every failing file passed when rerun on its own; in the second full run all six browser-pack files passed. No user-visible behavior, API or docs change.

[mrgoonie]

Review attestation: ready to merge at 08be431433ede86f781e009ee88d5e3f38fd35c7, reviewed by agent:code-reviewer.

A push to this PR makes this attestation stale; the new head needs its own review.

[mrgoonie]

Heads-up from #622 (#563): packs/browser-playwright/test/new-page-failure.spec.ts calls rmSync(dir, { ..., maxRetries: 10, retryDelay: 200 }). On Windows, under Node 24, that form fails with EPERM at once and never runs its retries. #622 adds the invariant test-cleanup-retries-asynchronously, which fails on that call. Whichever PR lands second should change it to await removeTestDirectory(dir) from tools/test-cleanup.ts, in an async afterAll.