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fix(flow): Errored builds a thunk fallback inside its own scope, like Show (#3624)

`<Errored fallback={() => <F />}>` is type-reachable (`() => X` satisfies
`(err, reset) => X`), but the runtime read `f.length === 0` as a value
thunk and handed it back unresolved, so the CONSUMING hole built it on the
enclosing owner's counter — the client at the statement, the server inside
the `ssr()` walk after every scoped sibling had reserved its slot. Aligned
only when nothing scoped followed the boundary; otherwise the keys
permuted (#3620 surfaced it and pinned the reported half).

Ruling: Errored's fallback works like Show's function child. Both runtimes
now call a function-valued fallback whatever its arity, inside the
boundary's own output scope where the two-arity form always ran, so both
arities allocate identically and nothing reaches the hole.

Audited Loading, Show, Switch, For, Repeat (fallback typed `Element`,
which excludes functions — type-unreachable, left to the #3620 ruling and
the UNSCOPED_HOLE_ALLOCATED_IDS check), Match and Reveal (no fallback).

Tests: #3567 harness `errored-thunk-fallback-followed-by-scoped-hole`
(server artifact → hydrate: nothing client-created, every `_hk` claimed,
no key miss, no diagnostic); the server spec's "reported when a scoped
hole follows" half flipped to "silent, aligned", plus zero- vs two-arity
id-layout parity; client specs for reactivity, owner placement, reset
through a rest-parameter fallback, and the two-arity form unchanged.

Size: hydrating app 20,907 → 20,981 B (+74 B brotli) with the minified
bundle 10 B smaller and an identifier-normalised diff of one statement —
esbuild's mangled-name reassignment shifting under brotli; cap 20.95 →
21.00 KB with a dated ledger note. With-stores −37 B, CSR −31 B.

Co-authored-by: Claude via Cursor <noreply@cursor.com>

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