fix: prevent browser workspace data loss

[Yuki9814]

Summary

• isolate corrupt and future-schema browser payloads without rendering or automatic overwrite
• classify missing, migrated, quota, unavailable, and cross-tab conflict states with accessible recovery actions
• preserve exact raw storage strings for lossless recovery and compare-before-write/remove checks
• keep failed tabs usable in memory with current-workspace backup, manual retry, and explicit reload/discard paths
• migrate historical schema v1–v7 workspaces safely to schema v8 without changing the storage key

User impact

PatchHive now fails visibly instead of silently replacing unreadable, newer, full, denied, or externally changed browser storage. A second tab that writes a newer or corrupt payload locks the first tab before normal save/import/reset; the first tab can back up its in-memory work and reload the stored value.

Web Storage has no atomic compare-and-swap operation. The exact re-read plus storage-event lock narrows the race window but does not claim strong multi-writer consistency.

Validation

• 106 unit and component tests passed
• 12 Chromium E2E tests passed, including real second-tab future-schema and corrupt-payload scenarios
• lint, TypeScript build, GitHub Pages path verification, dependency tree, and diff checks passed
• npm audit: 0 vulnerabilities
• 6 scanner benchmark scenarios passed on Apple M5 arm64; 250-finding p95 preview 36.68 ms JSON and 52.21 ms SARIF, with p95 heap 20.82 MiB and 21.33 MiB
• rendered desktop and 390x844 mobile checks: correct page identity, no blank/error overlay, no horizontal overflow, no console warnings/errors, and visible conflict lock actions
• two independent final reviews: no findings

Evidence status

Part of #13.

External usability evidence remains N=0 trials. This PR does not close the evidence issue or claim external adoption.