PLN, mid-cut: "you're hogging the system :(". Nine children at ~88% of a core each
put the load average at **15.6 on 16 cores**, and the machine he performs and edits
on became unusable. My fault twice over — the default was one subprocess per track
regardless of the box, and nothing told the scheduler this work was background.

Both fixed at the source rather than by remembering to pass a flag:

* `--jobs` defaults to **half the cores, capped at 6**. It is the total that matters,
  not the track count, and each child peaks near 2 GB.
* Children start at **nice 19 and SCHED_IDLE** via a preexec hook, so they only run
  when nothing else wants the CPU. The nine still saturate the sixteen cores; the
  editor now wins every scheduling decision, because nothing waits on this work but
  me. (The live run was reniced in place rather than killed — eight minutes of
  analysis is not worth a restart.)

The test for that hook is worth a note, because the first version of it was fake: it
looked the hook up through `_fan_out.__globals__`, where a nested function does not
live, got `None`, fell back to a no-op lambda, and passed. Same shape of mistake as
the finder test earlier today. So `run_at_idle_priority` is module-level now and the
test asserts on a **real child's** reported nice and scheduling policy — verified to
read `0 0` without the hook and `19 5` with it.

Also in here, from the same run: the presence gate I added to the finder was doing a
full `max(abs(window))` per candidate — at 8 bars that is ~700 k samples times
thousands of windows, which is most of why this run was slower than the last. It now
reads a per-beat peak table computed once per stem. Exact, not approximate: the
inter-beat segments partition `[int(times[i]*sr), int(times[e]*sr))` precisely, so it
visits the same samples — with a test that checks the two agree for every window
rather than trusting the argument.

And `kitgate --link` now takes each kit's staging root from the manifest instead of
`publish.samples_root()`. A pack cut with `--samples-root` lives beside its source, so
the default root would have looked in the wrong place and called every kit missing.

Suite 122 → 125.