test(signals): visibility oracle — posture matrix, and the postures brought to a ruling

[ryansolid]

Tests and spec only — no runtime change.

What

The visibility oracle pins node state × reader kind with every reader built mainline. This adds the reader's posture — where it lives when built — and a second observable, entanglement: release only the source's hold and see whether x reveals or is blocked by the reader's context.

Postures: mainline, foreignAction (built inside another live action's body), foreignLane (that action holds its own optimistic write), behindFallback (inside a createLoadingBoundary whose content is pending on a sibling flight), disposedReader (root disposed before the source's hold releases).

tests/visibility-oracle-posture.test.ts — 280 cells, discovery mode: every cell is recorded to VISIBILITY_POSTURE_REPORT as a markdown table; none pinned. The oracle's states move to visibility-oracle.states.ts so both runners share them; the existing runner is unchanged (126 cells, same results).

Rulings from the first pass — tests/posture-born-held-and-observation.test.ts

• A — same-tick adoption is by design (spec O1, ruled): transitions are ambient; an action is the special case that links one across async; every write is a conceptual transition. An action started later in the same tick owns the ambient writes made before it (16 cells).
• C — latest() entangles (A15, ruled): lanes are transition-bound, so latest(x) inside another live action merges that action with x's — the value pokes through at once, the reveal is joint (8 cells).
• B — creation under a transaction escapes the hold (spec O2, recorded, not ruled): a memo + render effect created inside a live action over a held value publishes the held value while untracked reads keep the committed frame (10 cells). Mainline creation is born held (A29); the every-posture extension was prototyped (+83 B, suite green) and set aside — "capture things as they are". Pinned as observed so a change flips loudly.

Recorded, not yet brought to a ruling

• Behind a showing fallback, isPending(x) answers false where mainline says true (held, superseded ×2, pending own async, body ended, held truth) — possibly A19 exception (1) applied to the reader's boundary.
• Boundary content under a live override reads the committed value, not the override (A17 says the override is the displayed value; hidden content may be "off the hold").
• disposedReader is clean in all 56 cells.
• Inside collection-boundary content, reads of pending nodes return undefined rather than throwing — the boundary's collection semantics, expected.

Why now

This is the file the fuzzer's (#3446) confirmed findings fold into as cells, and it gives O1/O2 a home on next for the triage to cite.

signals 2,369 green on the rebase.

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_{Comparing oracle/posture-matrix (a411dbf) with next (5662bbb)}