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fix(signals): seat a first pass in its creator's lane, never a verdict lane (#3835, #3851) (#3869)

* fix(signals): a binding an optimistic mount creates lands with its element (#3835)

A first pass created by a guess's lane is the lane's frame (ruling A) and
recompute's tail already routed its result and run there, but its head
reset `passLane` for every fresh node, so the pass read as the action's
own pass in the action's flush: served the held staging and joined the
transaction. Inside a memo the lane created, the binding was born held
while the memo's element showed through the lane (`<div>Hello</div>`
with no style until the action landed); directly under the lane pass it
showed the held value early. The head now seats a first pass in its
creator's guess lane, so the binding is the lane's stale reader of the
held write: committed value now, re-derived at the landing (frameRead,
2026-10-01: a lane sees the screen plus its own guesses).

Verdict lanes are not inherited: verdict work reads the staged world and
a mount it makes stays a mainline mount (A29's boundary exemption; the
#3540 keyed-Show-over-latest rows).

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

* fix(signals): a verdict reader's mount stays mainline (#3851)

Inside an action, `<Show when={latest(x) > 0}>` opens at once, and a
`<Loading>` it mounted showed `content 1` beside the committed `x = 0`.
Three gaps let the proposal through:

- #3835's seat excluded a verdict lane only at recompute's head; the tail
  still routed the first pass to the creator's lane (`passLane ??
  creatorLane`), so the content was staged as verdict-lane work. The
  creator's lane is now filtered once, for both.
- Mainline, the content's first pass ran in the action's own flush and
  read `x`'s staging before the seam held it: neither joined nor created
  by a joined pass, it published directly. A first pass that read a
  staging of a flush with a transaction is now born held like one that
  read a... (continued)

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