Summary

Restore browser writable-output-path hardening. Part of umbrella audit #2406 (B10 silent-revert pattern).

Upstream Commit

• 6a80e9db05 — fix(browser): harden writable output paths
• Sync batch: v2026.3.1
• Classification: Production — security hardening

Files to Restore

• src/browser/output-atomic.ts (A — new file in upstream, absent in fork)

Context

Missed during B10 sync (PR #2400) via the asymmetric revert-vs-extract split anti-pattern. File exists at upstream/v2026.3.22; the INCLUDE Reconciliation Gate in fork-sync SKILL § Phase D.3 now prevents recurrence.

Acceptance Criteria

• src/browser/output-atomic.ts present in fork main matching upstream/v2026.3.22 state
• pnpm check passes
• pnpm test passes
• PR cherry-picks 6a80e9db05 with -x to preserve upstream attribution

Execution

git cherry-pick -x 6a80e9db05
# Expect minor adaptations: rebrand transforms, import-path divergence
# Pattern reference: #2402

Related

• Umbrella: Audit: upstream fixes silently lost in sync (v2026.2.25 → v2026.3.22, 32 gaps) #2406
• Prior pattern: fix(ci): replace tlon git api dependency (closes #2362) #2402 (tloncorp/api fix)
• Prevention: INCLUDE Reconciliation Gate (fork-sync SKILL § Phase D.3)