feat(backend): T-4 — named sessions through the boilerroom wrapper

The flavor probe gains a third answer: the wrapper names itself on
/health, so behind OLLAMA_HOST tatlock now distinguishes boilerroom,
a bare llama-server, and Ollama. Through the wrapper each pipeline
phase is a named session with the decided eviction ranking —
tatlock-steward/-orchestrate/-synthesize at 40, librarian at 30,
lower parks sooner (webber will sit at 20; Open WebUI stays
session-less and can never evict anyone). Against a bare llama-server
the raw id_slot pins survive unchanged, Ollama gets neither, and an
unprobed flavor sends nothing rather than guessing — the backend
stays swappable by env alone. tool_choice through the wrapper follows
the llama-server rule, since that is who answers.

The wrapper's balancing and compaction-due signals are read
everywhere: an httpx response hook on the provider covers every
PydanticAI call, streams included, and the steward's raw call reads
the body extras. Acting on compaction_due is a future ticket — the
signal just must not pass silently.

Verified end to end against the live wrapper: the dev server probed
flavor=boilerroom, a full pipeline turn answered in 5.7 s, and
GET /sessions showed all three phase sessions resident at rank 40
with engine-reported occupancies. The demonstration also filled the
production slot map — session-less prod delegations would have 503d
— cleared by a wrapper restart and filed as boilerroom T-11 (sessions
need an exit). A latent test flaw surfaced too: the ollama
tool_choice test relied on the dev backend probing as ollama; it now
pins the flavor it claims to test.

27 selector tests (9 new), 677 total green; three mutations shown to
fail their tests (librarian rank, the wrapper branch, the no-nudge
set); the new wrapper contract class runs 10/10 against the live
boundary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.env.example

@@ -8,11 +8,16 @@ API_HOST=0.0.0.0
 API_PORT=8000
 API_PREFIX=/v1
 
-# Ollama Configuration (local - primary backend)
+# Local backend (primary). The var name is legacy: any OpenAI-compatible
+# server works — Ollama, a bare llama-server, or the boilerroom wrapper
+# (http://127.0.0.1:8602 on tower-of-joy). The flavor is probed at
+# startup; through the wrapper, BACKEND_SLOT_PINNING=true sends named
+# sessions with the T-4 eviction ranking instead of raw slot pins.
 OLLAMA_HOST=http://localhost:11434
 OLLAMA_DEFAULT_MODEL=gemma4:e2b
 OLLAMA_TIMEOUT=120
 STEWARD_TIMEOUT=60
+BACKEND_SLOT_PINNING=false
 
 # Anthropic Configuration (Claude - cloud fallback)
 # Set ANTHROPIC_API_KEY to keep the Claude fallback available: it is used