feat(signals): WIDE_WRITE attribution warning

[brenelz]

Follow-up to #3018: the written-fan-out warning, implemented per the review guidance — via the dev-maintained _subCount from the graph-size diagnostics, no core sites touched, specced together with HUGE_FAN_OUT so the two never double-fire.

What it catches

A committed root invalidation — sync write, refresh(), or async landing — reaching a node with ≥ 250 live subscribers (default). Where the per-scope warnings blame the reader, this one blames the write: the fan-out actually happening, priced at the moment it happens.

[WIDE_WRITE] write to "selectedId" reached 1000 subscribers — every one re-runs
this flush. If consumers ask keyed questions of this value (for example every row
comparing against one selected id), invert with createSelector or createProjection
so only the keys whose answer flipped update.

Reconciliation with HUGE_FAN_OUT

• HUGE_FAN_OUT: always-on, fires at link time from GRAPH_SIZE_WARN_AT (2000) up — structure so large it warns even if never written.
• WIDE_WRITE: attribution-only, fires at write time from a much lower bar (250), because fan-out only costs anything when the node actually changes. Once per node, re-warning only after the subscriber count doubles — so a node that trips both gets at most one WIDE_WRITE alongside HUGE_FAN_OUT's own milestones.
• Documented in documentation/solid-2.0/08-dev-diagnostics.md, with a Related: cross-reference under HUGE_FAN_OUT that mirrors the existing HUGE_FAN_IN ↔ WIDE_SCOPE_DEPS pairing.

Implementation

One placement decision worth a look: the check lives in the engine's stampWrite rather than directly in the write hook. stampWrite is the single funnel for all committed root invalidations (writes, refresh(), async landings), so one call site covers all three — and inherits a correctness property for free: setSignal only stamps after the equality gate, so unchanged writes (which notify nobody) can never warn. The message's verb (write to / refresh of / async landing on) and data.write carry the kind.

Tests

5 new in tests/attribution-wide-write.test.ts: warns once at threshold with muting on repeat writes; re-warns only after 2× subscriber growth (30 → 55 silent, → 65 warns); fires for refresh() of a wide memo; quiet under threshold and for equality-gated unchanged writes; opt-out. solid-signals suite green (1323).

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Latest commit: c4a2f2f

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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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