Fix stale state in callbacks when multiple events fire rapidly

[Madoshakalaka]

Description

The UseReducerHandle was caching a snapshot of state at render time.
When set() was called, the underlying RefCell was updated, but callback
closures still held handles pointing to old snapshots. This caused
stale state reads when multiple events fired before a rerender.

The fix changes UseReducerHandle to:

1. Store both a reference to the shared RefCell AND a snapshot
2. Use try_borrow() to read from RefCell when possible
3. Fall back to snapshot when RefCell is borrowed (during dispatch)

This ensures callbacks see the latest state in normal operation while
avoiding panics when state is read during reducer execution (e.g., SSR).

Changes:

• UseReducerHandle now stores both current_state RefCell ref and snapshot
• Deref uses try_borrow with fallback to snapshot
• Updated Clone, Debug, PartialEq implementations
• Added regression test for the stale state scenario

Fixes #3796

Checklist

• I have reviewed my own code
• I have added tests

[github-actions]

Benchmark - core

Yew Master

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.401 ns      │ 3.666 ns      │ 2.406 ns      │ 2.528 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.435 ns      │ 3.695 ns      │ 2.438 ns      │ 2.49 ns       │ 100     │ 1000000000

[github-actions]

Visit the preview URL for this PR (updated for commit 6f0b966):

https://yew-rs--pr3963-fix-stale-state-issu-iujte35q.web.app

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[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	98.371	98.371	0	0.000%
boids	168.064	168.064	0	0.000%
communication_child_to_parent	91.499	91.499	0	0.000%
communication_grandchild_with_grandparent	102.606	102.606	0	0.000%
communication_grandparent_to_grandchild	98.987	98.987	0	0.000%
communication_parent_to_child	88.859	88.859	0	0.000%
contexts	104.146	104.207	+0.062	+0.059%
counter	85.530	85.530	0	0.000%
counter_functional	85.895	86.039	+0.145	+0.168%
dyn_create_destroy_apps	88.698	88.698	0	0.000%
file_upload	98.330	98.330	0	0.000%
function_delayed_input	91.414	91.603	+0.188	+0.206%
function_memory_game	170.055	170.503	+0.448	+0.264%
function_router	329.168	329.847	+0.679	+0.206%
function_todomvc	161.833	162.228	+0.395	+0.244%
futures	236.338	236.338	0	0.000%
game_of_life	103.926	103.926	0	0.000%
immutable	246.370	246.486	+0.116	+0.047%
inner_html	80.014	80.014	0	0.000%
js_callback	107.929	108.117	+0.188	+0.175%
keyed_list	179.217	179.217	0	0.000%
mount_point	83.230	83.230	0	0.000%
nested_list	112.803	112.803	0	0.000%
node_refs	90.874	90.874	0	0.000%
password_strength	1742.563	1742.563	0	0.000%
portals	92.370	92.370	0	0.000%
router	302.281	302.399	+0.118	+0.039%
suspense	111.874	112.029	+0.155	+0.139%
timer	88.158	88.158	0	0.000%
timer_functional	96.204	96.447	+0.243	+0.253%
todomvc	141.639	141.639	0	0.000%
two_apps	85.394	85.394	0	0.000%
web_worker_fib	133.512	133.862	+0.351	+0.263%
web_worker_prime	185.091	185.527	+0.437	+0.236%
webgl	82.650	82.650	0	0.000%

✅ None of the examples has changed their size significantly.

[github-actions]

Benchmark - SSR

Yew Master

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	290.951	292.175	291.222	0.346
Hello World	10	521.577	526.683	523.342	1.721
Function Router	10	1724.673	1776.752	1742.831	15.514
Concurrent Task	10	1005.368	1007.183	1006.574	0.585
Many Providers	10	1108.893	1207.702	1139.584	28.007

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.602	311.460	310.958	0.251
Hello World	10	514.773	566.560	529.352	14.194
Function Router	10	1737.771	1782.760	1757.753	14.452
Concurrent Task	10	1006.219	1007.404	1006.843	0.391
Many Providers	10	1097.124	1143.373	1111.597	14.878