feat(gen-seat): swap to AEON-ULTIMATE NVFP4 W4A4; default reasoning_effort=medium

Operator call: the incumbent abliterated model was the first one we could
find, not an optimised pick. sakamakismile/Qwen3.8-27B-AEON-ULTIMATE-
UNCENSORED-NVFP4 (base AEON-7 BF16, abliterix-abliterated, Apache-2.0),
byte-verified at /tank/aimodels/qwen38-27b-aeon-ultimate-nvfp4.

Measured on the same harness, same GPU, cache-busted per playbook 5.
Baseline was RE-measured live before the swap rather than trusted:

                        incumbent (W4A4+FP8 mixed)   AEON (W4A4)
  decode bs=1           94.09 tok/s                 104.22   +10.8%
  MTP acceptance        47.7%                        52.3%    +4.6pp
  abliteration          4/4                          4/4
  surface               6/6                          6/6
  weights               22.5 GB                      20.6 GB  -8.4%

AEON concurrency: conc=1 98.48 tok/s aggregate; conc=6 381.29 aggregate /
63.55 per-stream, MTP holding 50.6% under load.

Surface 6/6 includes vision (image-judge rides this seat) and a 36k-token
needle retrieval, which was the specific risk in going full-W4A4 -- the
packager only validated 32k, and W4A4 long-context collapse is in our own
notes from the Granite work. It held.

reasoning_effort: the AEON template defaults to xhigh (template line 47),
and at xhigh this model can spend its entire budget inside <think> and
emit no answer -- a silent-empty-response hazard for the automated
summarizer/classifier consumers. Seat now pins the default to medium via
--default-chat-template-kwargs, per-request overridable. Override PROVEN
live: chat_template_kwargs.reasoning_effort=bogus returns HTTP 400
carrying the template's own exception text, so caller values genuinely
reach the template and invalid ones fail loudly rather than silently
falling back. Empty GEN_REASONING_EFFORT omits the flag for models that do
not read the kwarg -- the Qwen3.6 line ignores it entirely, where setting
it would be a false lever.

All 7 aliases verified routing. Rollback is one .env line; the previous
build is untouched at /tank/aimodels/qwen38-27b-uncensored-nvfp4-mixed.

TWO GAPS, declared:
  - Incumbent concurrency was never captured before the swap (I baselined
    bs=1 only), so the conc=1/6 figures have no same-hardware comparator.
  - Perplexity NOT measured. eval_quality correctly refused it: under
    --speculative-config prompt_logprobs come back ~uniform (median rank
    ~130k), playbook trap 2. A real PPL number needs both seats served
    without spec-decode.

Adds concbench.py (concurrent throughput; wall-clock aggregate, not
sum-of-rates, and delta-based MTP accounting).

stacks/gen-seat/compose.yaml

@@ -62,6 +62,21 @@ services:
       - '{"image": 4}'
       - --reasoning-parser
       - ${GEN_REASONING_PARSER:-qwen3}
+      # Seat-level DEFAULT reasoning effort, per-request overridable via
+      # chat_template_kwargs. The AEON-ULTIMATE chat template defaults to
+      # `xhigh` (template line 47), and at xhigh this model can spend its ENTIRE
+      # token budget inside <think> -- the block runs past 18k chars, sometimes
+      # degenerating into repeating one line, and no answer is ever emitted.
+      # That is a silent-empty-response hazard for the automated consumers on
+      # this seat (`summarizer`, `classifier`), so the default is pinned down a
+      # notch rather than left at the model's own.
+      # Template accepts ONLY xhigh | medium | low and RAISES on anything else,
+      # so a typo in a caller's chat_template_kwargs is a 400, not a silent
+      # fallback. Empty value = omit the flag entirely (pre-AEON models such as
+      # the Qwen3.6 line do not read this kwarg at all, and setting it there
+      # would be a false lever).
+      - --default-chat-template-kwargs
+      - '{"reasoning_effort": "${GEN_REASONING_EFFORT:-medium}"}'
       - --enable-auto-tool-choice
       - --tool-call-parser
       - qwen3_coder