feat(signals): UNSTABLE_MEMO_OUTPUT attribution warning

[brenelz]

Follow-up to #3018 (the "spec now" item from the warnings discussion): the fan-out amplifier detector.

What it catches

A memo that commits a referentially-new but shallowly-equivalent plain object/array on N consecutive runs (default 4) has an equality gate that never closes — every subscriber re-runs on every upstream change. A wide memo with fresh object output is the single worst fan-out machine in real apps, and until now was only findable by profiling.

[UNSTABLE_MEMO_OUTPUT] memo "view" produced a new-but-equivalent object on 4
consecutive runs — its equality gate never closes, so every subscriber re-runs
on every upstream change. Return stable references or pass an `equals` option.

Implementation — engine-side only, no new core hook sites

• recomputeStart snapshots the node's committed value into the existing per-run frame (mirroring recompute's own pending-aware prev resolution, since an earlier run in the same flush may still be holding in _pendingValue).
• The compare runs at recomputeEnd, not derivedChanged: that hook fires before the commit branches, when the new value is still a local inside recompute. By recomputeEnd it is observable via _pendingValue/_value, and the phase facts are in hand.
• Plain objects and arrays only. Promises, Dates, Maps, class instances have no representative own enumerable keys — a fresh Promise per run is a genuinely new value, not instability. Pinned by a test.
• Phase-aware: overlay (optimistic/transition) runs are excluded — a lane re-derive legitimately produces fresh equivalents while it settles.
• Key-capped at 64 so hot paths stay cheap; once per streak; a genuinely different value resets the streak (so a later regression warns again).
• Opt-out via enable({ unstableMemos: false }), threshold configurable.

One robustness fix surfaced by the tests: an explicit undefined in enable() options clobbers defaults through the spread, so the check treats any non-number as disabled rather than warning on run 1.

Tests

5 new in tests/attribution-unstable-memo.test.ts: warns once at threshold (runs: 4) for fresh-equivalent objects; arrays; streak resets on genuine change; silence for stable refs / primitives / Dates; opt-out. solid-signals suite green (1323).

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Latest commit: c383795

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

This PR includes changesets to release 9 packages

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

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