Refuse re-entering a finished transaction; attribute the flush loop guard (#3140)

[ryansolid]

Summary

Companion to #3143 (merged — @snowfluke's clear of _transition stamps at commit), completing #3140. Stacked on #3145.

• The belt: a dead _transition reference can still reach initTransition from outside the commit path — merged _done forwarding chains, async settles racing completion — and setSignal re-opens a node's stamped transaction before the value-equal bail, so re-activating the corpse spins the drain loop (dev threw the flush loop guard; production hung the tab). initTransition now refuses a transaction whose chased _done chain ends in true, as a bare return — the reporter measured that redirecting to a fresh batch re-arms the loop with a new identity each pass.
• Guard attribution (the reporter's explicit ask in 2.0.0-rc.4: flush() guard trips from the asyncWrite write-back on a screen with several async memos (no minimal reproduction) #3140): the dev loop guard now reports what kept the loop alive — scheduled work vs an active transition, the transition's done-state and queue counts, and the last staged node. The corpse signature reads done=true, pending=0. Dev-only, tree-shaken from prod.
• Regression pins, white-box since the wild stamping race did not minimize: commit clears the stamp (pins fix(signals): clear a node's transition stamp when its pending value commits #3143); a value-equal write to a dead-stamped node leaves activeTransition null (fails without the belt when a dead stamp survives by any path).
• Size/perf: the belt is one compare folded into the existing transaction-open check; no loops, no allocation. One size budget ratcheted 26.1 → 26.15 KB (~25 B shared across fix(signals): clear a node's transition stamp when its pending value commits #3143, Close the ambient transaction window when a transaction opens without writes (#3141) #3145, and this).

Test plan

• Resurrection pin fails with fix(signals): clear a node's transition stamp when its pending value commits #3143 alone (planted dead stamp), passes with the belt
• Full signals suite (1437) green post-rebase on fix(signals): clear a node's transition stamp when its pending value commits #3143; size budgets pass

[changeset-bot]

🦋 Changeset detected

Latest commit: fb08fe4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@solidjs/signals	Patch
test-integration	Patch
@solidjs/web	Patch
solid-js	Patch
@solidjs/babel-plugin	Patch
@solidjs/compiler	Patch
@solidjs/html	Patch
@solidjs/h	Patch
@solidjs/universal	Patch
@solidjs/element	Patch
@solidjs/diagnostics	Patch

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[ryansolid]

Landed directly on next as a cherry-pick (GitHub closed this PR instead of retargeting when its stacked base branch was deleted with #3145's merge). Content unchanged; signals suite green (1437 passed).