hiding the suite that said so

armada/api/engines/grade.py is a vendored copy of tools/foundry/engine/grade.py
and had fallen three fixes behind the canonical:

  * the `empty_dbfs` presence gate. Every other sub-score is undefined on
    silence — the seam between two silences is perfect, the DC of silence is
    zero, its zero crossings are trivially fine — so a silent window scored
    0.90 x perfect and came out tier A. Fourier is deployed; this is what it
    was serving.
  * the seam denominator: mean |2nd diff| collapses to ~0 on sparse material,
    so a genuinely clean dub-drum loop measured 129x the mean and read as a
    huge click. The canonical uses the 90th percentile, amplitude-floored.
  * `not empty` on the low-RMS and mono-incompatibility flags, so silence stops
    being reported as maximally mono-incompatible.

Re-vendored by copy after checking the diff both ways: the canonical is a strict
superset and the only vendored-only lines were the OLD versions of the three
changed ones, so nothing API-specific was lost. Now byte-identical, which is
what the drift guard actually wants.

The drift guard was not broken. It had been failing, in-repo, unseen — because
of the second half of this commit.

armada/api needs FastAPI, which lives in ~/.virtualenvs/fourier. Run `pytest`
from the repo root with the plain interpreter and its seven test modules raise
ModuleNotFoundError during collection, which pytest reports as "Interrupted: 7
errors during collection" and then runs NOTHING — not the api suite, and not
the other 890 tests either. The natural response is `--ignore=armada/api`,
after which the api suite has no verdict at the root at all and can sit red
indefinitely. It sat red long enough for the grader to drift.

So conftest.py now skips that tree when fastapi is absent and prints the exact
command to run it properly. An unrunnable suite should say it is being skipped
and say how to run it; an error that stops everything only teaches you to pass
a flag that hides it.

  plain python3 at root      892 passed,  2 skipped   (was: 0 run, 7 errors)
  fourier venv at root       993 passed,  2 skipped   — the whole workspace
  armada/api in its venv     101 passed,  0 failed    (was: 1 failed)