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fix(signals): release projection leaf nodes when their readers let go (#3355)

Every node materialized under a projection or derived-store family is
linked into the projection computed's firewall child chain (so a mark on
the derive reaches leaf subscribers). The chain was singly linked and
append-only: the unobserved sweep dropped a node from its target's cache
but never unlinked it, so a long-lived keyed record retained one node —
and the last value it served — per leaf ever read, until the projection
itself was disposed. The rc.1 rewrite kept the legacy store's drop-from-
map disposal but added this second reference that never let go.

The chain is now doubly linked (_prevChild on the signal literals) and
the four unobserved hooks (value, presence, key-set, deep witness) unlink
in O(1). Deleted rows and their nested objects are collectable while the
projection stays live, and markNode's per-mark child walk covers live
leaves only. Dan's repro: 12.2 MB -> 0.4 MB retained after delete-all.

Tests: structural chain-length tests for the keyed record, the jsfb
selection map, a rows projection, still-subscribed deleted slots,
presence/key-set/deep nodes, derived stores; a WeakRef heap test in
gc.test.ts. Size: +1 literal field per signal — four caps bumped.

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

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