config(gen-seat): reasoning_effort=medium default (affects gen-reasoning only)

Measured on the restored model: at the template default xhigh, reasoning
runs 4,529-5,532 chars on a 3-turn history and was observed spiking to
9,261; medium holds it to 2,602-3,283 with content length unchanged or
better. Per-request overridable; an invalid value 400s.

SCOPE CAVEAT, stated because I applied this while chasing the wrong path:
this is a NO-OP for the  alias, which sends enable_thinking:false and
produces zero reasoning. It affects  only. The operator's
reported multi-turn failure was on , so this does not address it.

stacks/gen-seat/compose.yaml

@@ -62,13 +62,36 @@ services:
       - '{"image": 4}'
       - --reasoning-parser
       - ${GEN_REASONING_PARSER:-qwen3}
-      # NOTE: a --default-chat-template-kwargs '{"reasoning_effort": "medium"}'
-      # line lived here during the 2026-08-16 AEON-ULTIMATE trial and was
-      # REMOVED on rollback, so that restoring the previous model restores the
-      # previous behaviour exactly and the model stays the only changed
-      # variable. Re-add it only alongside a template that actually reads the
-      # kwarg (AEON's does, at chat_template.jinja:47; the Qwen3.6 line ignores
-      # it entirely, where setting it would be a false lever).
+      # Seat-level DEFAULT reasoning effort. Per-request overridable via
+      # chat_template_kwargs; an invalid value 400s rather than silently
+      # falling back (the template validates and raises).
+      #
+      # WHY: the Qwen3.8 chat template defaults to `xhigh`
+      # (chat_template.jinja:47), and at xhigh reasoning GROWS WITH
+      # CONVERSATION DEPTH and has a long tail. Measured 2026-08-16 on THIS
+      # model (not inferred from a card), 3-turn history, 3 samples each:
+      #
+      #     xhigh (default)  4,529-5,532 chars, observed spiking to 9,261
+      #     medium           2,602-3,283 chars
+      #     low              1,657-2,505 chars
+      #
+      # Content length was unchanged or BETTER at medium (median 335w vs 343w),
+      # so this is ~40% less budget burned for the same answer. Single-turn
+      # probes do not surface it -- single-turn reasoning is only ~1-1.8k chars,
+      # which is why the acceptance gate missed it entirely.
+      #
+      # The failure it prevents: at xhigh the reasoning tail exhausts max_tokens
+      # before any content is emitted, returning finish_reason=length with an
+      # EMPTY content field. That reads as the model being degenerate on long
+      # conversations when it is a budget interaction. Pairs with the
+      # gateway-chat max_tokens default (raised 1024 -> 4096); NEITHER fix alone
+      # is sufficient.
+      #
+      # Only set this for a template that actually reads the kwarg -- the
+      # Qwen3.6 line (Fable-Fusion on char-rp-reasoning) ignores it entirely,
+      # where it would be a false lever.
+      - --default-chat-template-kwargs
+      - '{"reasoning_effort": "${GEN_REASONING_EFFORT:-medium}"}'
       - --enable-auto-tool-choice
       - --tool-call-parser
       - qwen3_coder