Both exist because CosmicFest is one stage-mic MP3, and the house pipeline
assumes twelve Ardour orbit-stems.

demucs_sections.py — per-track stems plus a global pass, PLN's ask: "to see how
both are interpreted locally and globally". One measured fact decides what that
comparison can show. htdemucs' receptive field is 7.8 s (model.segment = 39/5)
and it always splits internally, so a 63-minute file gives the model no more
musical context than a 4-minute one. The local/global difference is therefore
NOT context — it is input normalisation, since demucs normalises by the mean/std
of whatever you hand it. On this gig that is a real lever rather than a nitpick:
the set sits at -25.2 LUFS with 11.1 LU of range, so quiet sections get far less
gain under whole-set statistics than under their own.

So global mode computes the normalisation over the entire recording and then
applies the model in bounded chunks with those statistics. That isolates the one
variable that can actually differ, in about 1 GB instead of the ~12 GB a single
63-minute tensor needs (four sources x 1.33 GB of output, plus apply_model's
overlap accumulator, against 12 GB free). Chunks overlap and are crossfaded with
an equal-power ramp — the seam is two estimates of the same audio summed, so a
linear fade would dip 3 dB in the middle of it.

Written against demucs' Python API rather than its CLI, because the CLI cannot
save here at all: torchaudio 2.10 routes writes through TorchCodec, which is not
installed, so `python -m demucs` separates for 87 seconds and then dies on
ImportError. Going through apply_model and soundfile also avoids installing
anything into PLN's venv to work around it.

Validated before launching a six-hour job: 30 s slice, four stems written, all
populated, and sum-minus-original sits 27.4 dB below the signal. Vocals came
back at -50.6 dBFS RMS on an instrumental passage — correctly near-empty, which
is the kind of agreement that makes later disagreements worth trusting. Runs as
a systemd --user unit per reference_durable_background_jobs, niced with idle IO
so it can never be why the rig stutters.

master_stemless.py — MASTERING.md's chain with the per-stem half removed and one
stage added, because the measurement demanded it. Full-file analysis first:
L/R correlation 0.833 (genuinely stereo, so bass-mono is safe), sub-30 Hz is
0.07% of total energy (the HPF is free headroom), and then the finding that
reshapes the job — 300-1000 Hz carries 34.75% while 4-8 kHz has 1.75% and
8-16 kHz has 0.12%. That is a microphone in a room, not a desk feed: the PA's
treble never reaches the mic, the room absorbs what does, mid-bass piles up. The
tape is dark and boxy, not merely quiet, so the dominant move is a broadband
tilt rather than gain.

Restraint where the numbers cannot justify enthusiasm: the air shelf is +5, not
the +12 the deficit suggests, because a 320 k MP3 puts codec residue in that band
alongside cymbals and a big boost lifts hiss into the master. The demucs stems
are the lens that can tell those apart; until they answer, the shelf stays modest.

Bass-mono runs in mid/side, not by splitting and re-summing bands: a lowpass(120)
summed with a highpass(120) leaves a phase notch at the crossover, whereas
high-passing the side channel alone never filters the mid path.

Two failures worth keeping. First, the club target with the limiter placed before
any gain: the signal reaching it still peaked at -9.5 dBTP, so a -1 dBFS ceiling
never engaged, and loudnorm's linear=true — correctly refusing to breach TP —
capped the master at -11.2 LUFS against a -9 target. The gain has to come first
so the limiter has something to catch. Second, that fix alone still undershot,
because every dB the limiter absorbs is a dB the final loudnorm cannot add; the
shortfall IS the missing gain, so feeding it back converges. Bounded at three
attempts and guarded by an LRA floor, since a loudness target reached by
flattening the music is not reached — presence_is_a_precondition, applied to
dynamics.

On the 90 s trial the streaming target lands at -14.0 LUFS, peak -1.0, with
1.8 LU of range lost. The club target stops at -10.5 with the floor tripped, and
reports the miss instead of clipping its way to a number. Whether -9 is
reachable on the full set, whose LRA is 11.1 rather than the slice's 4.6, is a
question the full render answers.