fix(gateway-chat): raise max_tokens default 1024 -> 4096; document single-file bind-mount trap

Operator reported the gen seat 'cutting off'. It is not the seat. The chat
UI's max_tokens field defaults to 1024, and every thinking seat spends part
of that budget on CoT before emitting content, so the completion truncates
mid-sentence with finish_reason=length and reads as model degeneracy.

Measured through the gateway:
  gen            1024 -> finish=stop, 716w   (survives, but marginally)
  gen-reasoning  1024 -> finish=length, cut mid-word  <-- the symptom
  gen-reasoning  4096 -> finish=stop, 839w

Seat itself is clean: direct long-form generations return finish=stop with
complete sentences and a max repeated 6-gram of 1 (no degeneration), and
enable_thinking:false still holds on every non-thinking alias, so the
AEON swap did not cause this.

Also documents a trap that made the fix look like it had not applied:
compose bind-mounts a single FILE, and a single-file bind mount binds the
INODE. rsync writes-and-renames, producing a new inode, so the container
kept serving the old content while the host file showed the new value --
silently, with no error. docker restart does NOT clear it; the container
must be recreated. Verify against what the container sees, never the host
file. Applies to any file-source mount; directory mounts are unaffected.

stacks/gateway-chat/conf/index.html

@@ -65,7 +65,13 @@
     <textarea id="sys" placeholder="You are a helpful assistant."></textarea>
     <div class="row">
       <div><label>Temperature</label><input id="temp" type="number" step="0.05" value="1"></div>
-      <div><label>Max tokens</label><input id="max" type="number" step="1" value="1024"></div>
+      <!-- 4096, not 1024: every thinking seat on this gateway (gen-reasoning,
+           char-rp-reasoning) spends part of the completion budget on CoT before
+           emitting any content, so a 1024 cap truncates mid-sentence with
+           finish_reason=length and reads as the MODEL being degenerate. Measured
+           2026-08-16: gen-reasoning at 1024 -> finish=length cut mid-word; at
+           4096 -> clean stop at 839 words. -->
+      <div><label>Max tokens</label><input id="max" type="number" step="1" value="4096"></div>
     </div>
     <label style="margin-top:.5rem">🔊 TTS (quoted text)</label>
     <label class="hint" style="display:flex;gap:.45rem;align-items:center;margin:.15rem 0"><input id="ttsOn" type="checkbox" style="width:auto"> Auto-play quoted dialogue</label>