docs(web): SAFETY comment for DerivedStateActor

[intendednull]

Why

Audit AUD-1: bare unsafe impl Send on DerivedStateActor no SAFETY comment. Future maintainer no clue if impl redundant or load-bearing.

Fix

• Add // SAFETY: block above unsafe impl Send in crates/web/src/state_bridge.rs.
• Comment walks four fields: StateRef<T> Send via params, Arc<dyn Fn ... + Send + Sync> Send via trait-object bound, Arc<Mutex<Option<U>>> Send via params, SendWrapper<WriteSignal<U>> unconditional Send w/ runtime cross-thread panic.
• Calls out single-WASM-thread invariant (Leptos browser-only) so SendWrapper runtime check never trips.
• Link back to spec follow-up F2 (docs/specs/2026-04-26-state-management-model-design.md) since cached Mutex tracked there.
• Did NOT delete impl — kept as guard so future !Send field forces reaffirmation.

Verify

• cargo fmt clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo test --workspace all pass
• cargo check --target wasm32-unknown-unknown clean for all WASM crates

Refs #435

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Generated by Claude Code