feat(summarizer): move local fallback off koala to iguana/gemma4-26b

koala now carries other GPU loads, so the first fallback should not run there.
Change the default chain to koala/phi4-mini → iguana/gemma4-26b → berget/mistral-small:
the local fallback now runs on iguana (M2 Ultra headroom, different host = different
egress IP for the rare fallback fetch). gemma4-26b is the brain-validated homelab
general-purpose model (agentsquad H2/H3 executor) and returned valid summary JSON on
the real prompt in a smoke test (~37s incl. cold-load — fine for a fallback path).

Pure config default (TAPIR_FALLBACK_MODEL); chain mechanism (ADR-022) unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/homelab-integration.md

@@ -30,7 +30,9 @@ it** — endpoints and aliases drift, and this file is a snapshot (2026-06-06),
 - **Summarizer fallback chain (ADR-022).** The primary alias is the *first* of an ordered chain;
   on failure or unparseable output the summarizer advances to the next model. All reached through
   the same gateway by alias.
-  - `TAPIR_FALLBACK_MODEL` — local fallback. **Default `koala/phi4-14b`.** Empty disables it.
+  - `TAPIR_FALLBACK_MODEL` — local fallback. **Default `iguana/gemma4-26b`** — on iguana, NOT
+    koala, so the fallback does not compete with koala's other GPU loads (and runs from a different
+    egress IP). Empty disables it.
   - `TAPIR_CLOUD_FALLBACK_MODEL` — worst-case EXTERNAL fallback. **Default `berget/mistral-small`.**
     **Set this empty (`""`) for any client/NDA deployment** so content never leaves the local
     stack — the chain then contains only local endpoints.