fix(signals): a released projection leaf leaves its firewall's companion set (#3503)

[brenelz]

Fixes #3503.

markFirewallChildCompanions registers a leaf on its projection's permanent _companionChildren set the first time it is read through latest() or isPending(). When the last reader is disposed, the unobserved sweep removes the leaf from the store's node cache and unlinkFirewallChild takes it off the firewall child chain (#3351), but nothing removed it from the companion set. Later writes address a fresh leaf, so the obsolete leaf, its last value, and its companion signal/computed stayed reachable from the live projection.

The fix removes the leaf from the set inside unlinkFirewallChild. The set exists only so the snap can iterate the children someone asked verdicts of; a leaf the store no longer addresses can never be asked again, so dropping it changes no verdict. Under __TEST__ the INV-4 companionOwners registry is released at the same point so the in-suite GC test can observe the runtime behaviour (the issue notes this registry would otherwise mask it).

Reproduced the issue's script (ordinary / latest / isPending / nested, delete and overwrite variants) against the prod build on next 191a572: ordinary passes, the rest fail; all pass with this change.

• Pin: packages/signals/tests/gc.test.ts (two cases, run with NODE_OPTIONS=--expose-gc npx vitest run tests/gc.test.ts; the existing should gc effect lazily case is broken on next independently and pnpm test:gc no longer finds the file under vitest 4).
• Size: signals: + createStore cap 16.45 -> 16.50 KB (measured 16,477 B; next measures 16,426). Core floor and + isPending/latest unchanged.
• Full signals suite: 197 files, 2736 passed.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 6e9243c

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
@solidjs/babel-plugin	Patch
@solidjs/compiler	Patch
@solidjs/diagnostics	Patch
@solidjs/element	Patch
@solidjs/h	Patch
@solidjs/html	Patch
solid-js	Patch
@solidjs/universal	Patch

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[codspeed]

Merging this PR will not alter performance

✅ 175 untouched benchmarks

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_{Comparing brenelz:fix/companion-leaf-release-3503 (6e9243c) with next (edafccb)}