Fast renderer: retry an effect that raised, like the default renderer

[maartenbreddels]

Part of the audit of REACTON_FAST=1 against the default renderer. This is one small PR per finding; this one is F2.

Problem

When an effect raises, Effect.executed stays False, and the default renderer runs it again on the next tree walk. The fast renderer skips a component whose element and state did not change (the clean-subtree fast path). So a failed effect in the root component, or in a memoized child, was never retried in fast mode.

Fix

The clean-subtree fast path in _RenderContextFast._render now also requires that the context has no effect work left: no effect that was never executed, and no pending next effect. The check loops over that component's own effects, so it is cheap. The benchmarks do not change: leaf update 0.65 ms, memo subtree skip 1.20 ms. The default renderer is not touched.

Tests

• test_failed_root_effect_is_retried_on_later_tree_walk and test_failed_effect_in_memoized_child_is_retried_on_later_tree_walk fail on master with REACTON_FAST=1.
• test_effect_cleanup_exception_does_not_retry_cleaned_effect checks that a cleanup that raised does not cause an extra retry.
• With this PR, all three pass in both modes.
• Full suite: 212 passed (default), 213 passed (fast).

The fix was written by a codex worker and verified by me. It is small, so it got no crossreview.

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