fix(parakeet): move the seat from the empty GPU 3 to GPU 0

Placed on GPU 3 first because it was the empty card. That was the wrong read:
the seat is ~800 MiB, under 1% of a 96 GB card, so the question was never "where
does it fit" but "whose headroom is cheapest to spend".

vLLM sizes its KV cache as a fraction of TOTAL VRAM, not free VRAM. A resident
tenant on an otherwise-clean card therefore does not cost its own megabytes — it
costs the profiling margin of whatever full-size seat lands there later, and
flash-next needs 93 GiB of 96. A 96 GB card at 2 MiB can still take that; the
same card at 922 MiB is one where the next big seat needs its utilization
hand-trimmed, which this repo's flash-next history shows is both thin and silent
when it goes wrong.

Committed utilization per card is the number that governs, not free bytes:

    GPU 0   0.40 + 0.48                       = 0.88    ~13 GB spare  <- moved here
    GPU 1   0.52+0.24+0.10+0.055+0.03+0.03    = 0.975   ~4.3 GB
    GPU 2   0.96                                        ~1.8 GB
    GPU 3   -                                           kept empty as reserve

GPU 3 is back to 2 MiB / 97,247 MiB free and is now documented as a deliberate
reserve rather than a spare.

Post-move n=5 on the same clip: 0.68 / 0.54 / 0.54 / 0.52 / 0.53 s, median 0.54 s
against 0.50 s on GPU 3. The spreads overlap at this sample size and no difference
is claimed; the GPU 3 figure was taken on an idle card and is now noted as a best
case, since the seat shares GPU 0 with the hot serving path. Silence control and
the gateway round-trip both re-verified after the move.

Also records both Parakeet endpoints (FV v3 on Blackwell, IRV v2 on a 3090) and
the four confounds that make them not an A/B pair, sent to tts-dev for the bench.

stacks/parakeet/compose.yaml

@@ -6,16 +6,23 @@
 # prebuilt int8 quantized Parakeet-TDT from k2-fsa — and wrote our own ~50-line
 # wrapper we own end-to-end.
 #
-# HOST: fv-ml1, GPU 3 (relocated from irv-ml1 2026-09-15). GPU 3 is the utility
-# card — the other three carry the vLLM serving seats and run 85-98% full, so a
-# seat placed anywhere else would fight them for VRAM.
+# HOST: fv-ml1, GPU 0 (relocated from irv-ml1 2026-09-15).
 #
-# ⚠ GPU pin is `deploy.resources.reservations.devices[].device_ids`, the fleet
+# ⚠ GPU 0, NOT the empty GPU 3. This seat holds ~800 MiB — under 1% of a 96 GB
+# card — so it rides on the card with the most uncommitted headroom rather than
+# putting the first fingerprint on a pristine one. GPU 0's seats commit
+# util 0.40 + 0.48 = 0.88, leaving ~13 GB; GPU 1 is at 0.975 and GPU 2 at 0.96,
+# both too tight. Keeping GPU 3 at 2 MiB means it can still take a full-card seat
+# (flash-next needs 93 GiB) without a neighbour eating its profiling margin —
+# vLLM sizes KV cache as a fraction of TOTAL VRAM, not free VRAM, so a resident
+# tenant on an otherwise-empty card is worth more than its megabytes suggest.
+#
+# ⚠ The pin is `deploy.resources.reservations.devices[].device_ids`, the fleet
 # convention — NOT `runtime: nvidia` + NVIDIA_VISIBLE_DEVICES, and NOT
 # `count: all` (which is what the dead on-host stub did, and would have let this
-# tiny ASR seat see all four cards including the three that are full).
-# device_ids ["3"] presents that card as cuda:0 INSIDE the container, which is
-# what sherpa-onnx's CUDAExecutionProvider takes by default.
+# tiny ASR seat see all four cards). device_ids presents the pinned card as
+# cuda:0 INSIDE the container, which is what sherpa-onnx's
+# CUDAExecutionProvider takes by default.
 #
 # Model weights (~460 MB int8) download on first run via the entrypoint to
 # ${PARAKEET_MODELS_DIR}/ (persistent host bind mount). Subsequent starts skip
@@ -51,7 +58,7 @@ services:
         reservations:
           devices:
             - driver: nvidia
-              device_ids: ["${PARAKEET_GPU:-3}"]
+              device_ids: ["${PARAKEET_GPU:-0}"]
               capabilities: [gpu]
     networks:
       - tnet
@@ -67,7 +74,7 @@ services:
       - homepage.group=AI - Audio Tools
       - homepage.name=Parakeet ASR
       - homepage.icon=mdi-microphone
-      - homepage.description=Parakeet-TDT speech-to-text via sherpa-onnx (fv-ml1 GPU 3)
+      - homepage.description=Parakeet-TDT speech-to-text via sherpa-onnx (fv-ml1 GPU 0)
       - homepage.href=http://10.251.50.54:${PARAKEET_PORT}
 
 networks: