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perf(signals): the plain flush skips the seam's effect-queue merge (#3790)

* perf(signals): the plain flush skips the seam's effect-queue merge

`settle()` collects three groups of runs apart — the lane reveals', this
flush's own, the landings' — and ordered them with two `concat`s every
flush, after two `[[], []]` pairs and two `length = 0` resets that ran
whether or not anything was in them. On the plain flush (nothing parked,
no reveal, no landing) all of it was a copy of `own` into a fresh array:
~125 ns of fixed cost per flush, 59% of it the merge.

- The merge keeps `own[i]` as the queue when there is nothing to order
  around it; every other seam builds the ordered queue as before (lanes,
  then own unless parked, then the landings' and anything a commit queued).
- A parked flush stashes its runs with `append` (in place) instead of a
  `concat` copy — no alias of a transaction's queue exists anywhere.
- `heldTrims` resets once after both branches and only when non-empty;
  `stagedReaders` likewise. `length = 0` is a runtime call even on an
  empty array (~15 ns each).
- `run()` reads its queue once.

Measured on the built prod dist, interleaved A/B against origin/next,
fresh process per run, medians: store setter delete+set one root key
(20k keys) 844 -> 748 ns/commit (-11%); projection root write 1244 ->
1167 (-6%); nested write 746 -> 660 (-12%); one signal + one render
effect + flush 240 -> 133 ns (-45%); 1000 signals + flush, dbmon full
tick and update1to1 within noise (interpreter mode -0.3%).

Size: +5 B minified in the core; brotli moves each scenario -45..+67 B
by layout (four frozen caps are over, see the next commit).

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

* size: three caps to measured + 10 B for the settle fast path

The seam change is +5 B minified wherever the signals core is retained;
brotli layout moves the scenarios -16..+28 B against `next` @ 924d909d9.
Three caps end up over: the simple-app floor (9,812 B, +11, 2 ... (continued)

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