feat: Add Handle::update_tracked for conditional rebuilds

[BinaryMuse]

Summary

• Adds TrackedRef<'a, S>, a borrowing dirty-tracker that wraps &mut S — reads through Deref/read() don't set the dirty flag, writes through DerefMut do
• Adds Handle::update_tracked() which only triggers a rebuild if the callback actually mutated state
• Handle::update() is unchanged and always triggers a rebuild

Motivation

In apps with expensive view functions, high-frequency state updates that don't actually change values (e.g., an input handler setting is_input_blank = false when it's already false) still trigger full rebuilds. update_tracked lets callers opt into conditional rebuilds:

handle.update_tracked(|state| {
    let new_blank = text.is_empty();
    if state.read().is_input_blank != new_blank {
        state.is_input_blank = new_blank;
    }
});

Test plan

• All 211 existing tests pass
• Manual testing in atuin-ai TUI with rapid keystroke input

[greptile-apps]

Greptile Summary

Adds TrackedRef<'a, S> and Handle::update_tracked() to enable conditional rebuilds — the callback only marks the app dirty if it actually writes through DerefMut. The implementation is correct: UpdateStateTracked is handled in all three dispatch paths, and the Deref/DerefMut split behaves exactly as documented.

Confidence Score: 5/5

Safe to merge — no logic issues found

All three message-dispatch paths handle the new variant, TrackedRef's dirty semantics are correct, and the only rough edge (remainder still using the verbose guard in renderer.rs) is a pure style inconsistency with no behavioral impact.

No files require special attention

Important Files Changed

Filename	Overview
crates/eye_declare/src/component.rs	Adds TrackedRef<'a, S> — borrowing dirty-tracker with correct Deref/DerefMut split; clean implementation
crates/eye_declare/src/app.rs	Wires UpdateStateTracked message through all three dispatch paths (two select! arms + drain_updates) via apply_tracked_update helper; no gaps found
crates/eye_declare/src/renderer.rs	Replaces manual divide-by-zero guard with checked_div for per_fill; adjacent remainder calc left unchanged (minor inconsistency, no bug)
crates/eye_declare/src/lib.rs	Exports TrackedRef alongside Tracked; straightforward
docs/content/guide/application.md	Adds clear documentation and example for update_tracked; example correctly guards writes with a read-first comparison

_{Reviews (1): Last reviewed commit: "Clippy" | Re-trigger Greptile}