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fix(signals): eager adoption no longer aliases swapped or shifted raws (#4216)

* fix(signals): eager adoption no longer aliases swapped or shifted raws (#4001)

descend paired slot x's row with the raw that moved in from slot y, which
repointed that raw's lookup to the row; slot y then found the row through
it and paired it again, so two slots read one row after the commit. The
same held for unkeyed reorders and shifts ([A, B, C] -> [B, C, D] collapsed
every proxied slot onto one row), for keyless rows of a keyed array, and
for a plain store's unkeyed reconcile.

descend now also looks up the incoming raw and keeps each row's `pk` on the
slot it sits at: a raw that is already a live row keeps it at the slot it
moved to, a row whose raw is at its `pk` slot of the incoming frame is not
paired again, and a raw still resolving to a row that moved on drops the
lookup and wraps fresh. One lookup per descended pair, no allocation.

createStore +69 B brotli / +176 B minified; compiled floor unchanged.

Fixes #4001

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015hPxMkC3jg3Uivh65gvaf2

* perf(signals): shrink the #4001 fix and offset it in the store code

The fix's descend reads the family from the parent target instead of a
fifth argument and drops a check every call site already makes. The rest
comes out of retained store code, behavior unchanged:

- notifyWrites' accessor-key diff is notifyKeyDiff's (one copy); the
  adoption callers' probe flag is the default
- sameLogicalSlot folds into targetsEqual (optional family argument)
- materializePB and flattenOverlay share overlayOnto; foldTarget closes
  the staging once; the privatized-mid-batch copy uses copyOwn;
  scanAccessorsOnce uses isOwnAccessor
- the set trap writes an unsafe key and an overlay miss through one
  defineProperty; the write traps' gate drops the derived `override`
- one deep-witness flag per adoption instead of three... (continued)

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