[loop cycle 4] fix(notifications): de-duplicate burst-repeated native toasts

[karem505]

What

Stops the same notification showing up two or three times. WhatsApp can raise the same alert more than once in a quick burst (a React re-render, or one message surfacing through two notification code paths). A browser collapses those by tag, but a native OS toast stacks every call as a separate visible popup.

How

Add a nativeNotify(title, body) helper that suppresses an identical title+body seen within a 3500ms window (pruning stale keys each call so the map stays bounded), and route the window.Notification shim through it.

• Distinct messages — including different messages from the same chat — are never suppressed.
• An identical alert is allowed again once the window expires.
• Title/body coercion is unchanged; lock/settings suppression still happens downstream in the Rust notify command.
• Map is Object.create(null) (no prototype-key hazards).

Verification

Gate 1 — cargo build --locked + cargo test: PASS (51 tests; bridge embedded via include_str!).

Gate 2 — real WebKitGTK engine harness (origin spoofed, invoke recorder): PASS

• 5 fires, 2 identical immediate duplicates → 3 native notifies
• distinct title/body messages all delivered
• the same alert re-fires after the 3500ms window expires (4 total)

Gate 3 — generation-blind code review: APPROVE, severity none, no must-fix. Confirmed dedup correctness, Object.create(null) key safety, safe for..in+delete prune, strict-mode legality, behavior parity.

🐞 A latent crash was caught and fixed during this cycle. An earlier iteration used a NUL byte as the dedup-key separator. JS string literals can contain NUL, so node --check passed — but the script is handed to WebKit as a C string, so the NUL truncated the entire injected bridge.js at that line (which would have silently broken drag-drop, unread counts, and notifications in the real app). The real-engine harness surfaced it; the separator is now a plain space and the file is pure ASCII.

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🤖 PR-ONLY — do not auto-merge. Releasing whatRust is manual via a v* tag; this loop never merges, bumps the version, or tags a release. Opened by the whatrust-fix-loop (cycle 4/6).