docs(ui): add /// doc comments above 19 impl blocks (R2.1 compliance)

[eastspire]

Summary

Add a single /// doc-comment line above every impl block across six hook modules so the impl block itself satisfies the rust-standards R2.1 rule (fn and impl blocks must have a /// doc comment directly above).

fn blocks were already 100% compliant. This PR closes the gap on impl blocks only — 19 insertions across 6 files, zero code changes.

What changed

File	impl blocks annotated
ui/src/hook/debounced_value/impl.rs	2 (inherent + Display)
ui/src/hook/lazy/impl.rs	3 (PartialEq for LoadState, inherent + Debug for LazyComponent)
ui/src/hook/previous/impl.rs	3 (inherent + Display + Default for Previous)
ui/src/hook/suspense/impl.rs	4 (inherent + Default + Display for SuspenseHandle, PartialEq for SuspensePhase)
ui/src/hook/throttled_value/impl.rs	2 (inherent + Display for ThrottledValue)
ui/src/hook/use_async/impl.rs	5 (Drop, slot-lifecycle inherent, public-API inherent, Debug, Default for UseAsyncHandle/UseAsyncSlot)

Why

rust-standards skill R2.1 mandates that every fn and every impl block in production code carries a /// doc comment that opens immediately above the item. audit_rust_standards.py does not enumerate this check (it covers 14 separate lints), so the audit tool's "all green" output masked the gap. A targeted scan (fn + impl regex + "block immediately above has ///") surfaced 19 impl blocks with no doc line — all in the hook layer.

Verification

• cargo check -p euv-ui passes (native).
• cargo check -p euv-ui --target wasm32-unknown-unknown was already green on master; no API change, so still green.
• euv fmt reports Formatted 0 file(s), 796 unchanged. — formatting holds.
• Re-scan after this change: Missing fn /// = 0, Missing impl /// = 0 across ui/src.

Scope / non-goals

• No code, signature, semantic, formatting, or version changes.
• No Cargo.toml edit, no version bump. euv-ui stays at 0.16.1.
• No new tests added — pure doc-comment additions cannot regress runtime behaviour, and the macro-aware comment-stripped test extraction in this crate already exercises the hook layer that wraps these impls.

Followup

A second PR is in progress to wire the hook functions into component-level consumers (euv_field → UseForm, animation primitives → UseTransition, etc.). That work is intentionally kept separate so this PR can land as a zero-risk compliance patch.