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fix(signals): a removed own __proto__ reads undefined, not Object.prototype (#4672) (#4676)

* fix(signals): removed own __proto__/constructor data reads undefined, not the prototype (#4672)

A store whose own `__proto__` (or `constructor`) data was removed told the
key's node the backing's inherited value, so the hot read served
`Object.prototype` wrapped as a store (or `Object`) to tracked and untracked
readers. Core R30: inherited pollution keys are never served. Now the node
is told `undefined`, the same answer a store that never had the key gives:
- notifyWrites (setter delete, a projection's draft delete): an absent
  pollution key's node gets `undefined`. An own key pays nothing, because
  the existing hasOwn result is reused.
- reconcile's removed-key branch (reconcile, and a derive's returned value):
  a pollution key diffs against an empty record.
- the get trap's existing pollution-key branch: a tracked reader subscribes
  like any absent key (R12), and its node is born absent. Own data that
  lands later reaches the reader (F-10).

`__proto__` assignment stays inert (R30).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWNSSgtemsSYsaX2hXE

* fix(signals): narrow #4672 to __proto__ after #4674

With #4674 on next, a plain read of an inherited `constructor` returns the
source's class, and a removed own `constructor` already reads it, as an
object that never had one does. Ruling: narrow the fix to `__proto__`.

- notifyWrites and reconcile's removed-key branch tell a removed
  `__proto__` node `undefined`. An own key does no extra work in
  notifyWrites (the existing hasOwn result is reused), and the reconcile
  check sits inside the existing removed-key branch.
- Dropped: the get-trap subscription and getNode's `absent` parameter (the
  reverse direction is pinned `it.fails`), and every constructor-specific
  change and test, apart from one pin that a removed own `constructor` rea... (continued)

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