Fast renderer: clean up removed components before new effects run

[maartenbreddels]

Part of the audit of REACTON_FAST=1 against the default renderer. This is one small PR per finding; this one is D5.

Problem

When a component replaces another one in a container (a conditional child, or a changed key), the default renderer runs the effect cleanups of the removed component before the effects of the new one. The fast renderer removed stale elements only after it had reconciled the whole context, so the new effect ran first. Two things went wrong:

• An effect that sets a global value and resets it in its cleanup ended with the reset value (None) in fast mode, but with the new value in default mode.
• When two cleanups raised, a different exception reached use_exception.

Fix

• The fast renderer now runs the effect cleanups of stale components at the same point in the walk as the default renderer, in sorted key order, including nested components.
• Their widgets are still closed later, after the parent got its new children. This is the fast renderer's existing, deliberate difference (D5c in benchmarks/README.md).
• The default renderer is not changed. It never reads the two new ComponentContext fields.

Tests

• Two new tests in core_test.py. Both fail on master with REACTON_FAST=1 and pass in both modes.
• fuzz_test.py now also logs effect cleanups. Before, it compared effect runs only, so it could not see this class of bug. With cleanups logged, 7 of its 12 cases fail on master; with this PR all 12 pass. A reviewer's extended fuzz run found 0 of 400 seeds that differ, against 217 on master.
• Full suite: 211 passed (default), 212 passed (fast).

Review

Crossreview with Astra (GPT-6), Opus and GLM, in 2 rounds. All three approve the final version.

• Round 1 found no ordering bug. It found:
  • pop(0) made the cleanup quadratic;
  • the new fields were not freed on close;
  • self.context was not restored;
  • the fuzz test could not see cleanups.
• All of these are fixed in the second commit, and round 2 confirmed the fixes.
• Open and not blocking:
  • If reconciliation aborts with an internal error between the early cleanup and the removal, a component can stay mounted with its effect cleaned up. A reviewer could only trigger this by injecting an internal error.
  • .shared() elements are still broken in both renderers, as they are on master. That needs its own issue.

The fix was written by a codex worker and reviewed and adjusted by me.

🤖 Generated with Claude Code