Add the per-host-model limiter and the host chooser

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

docs/implementer-log.md

@@ -14,6 +14,7 @@ owner fills in the Model column. The reviewer adds findings under "Reviews" once
 | v0/04-admin-main | 2026-09-25 | done | 1 | pass | none | `timeout --signal=TERM 3` exits 124 on a timed-out child on this GNU system, so the task's `exit=0` is not observable through it; sent SIGTERM directly and confirmed crossbar's own exit code is 0 with both log lines. | llama.cpp/ornith-1.5-35b-a3b |
 | v0/05-smoke-readme-deploy | 2026-09-25 | done | 1 | pass | none | `README.md` `## Run` uses `install -m` instead of `cp` and adds `systemctl daemon-reload` before `enable --now`, which is required for systemd to see the new unit; the task said only "copy … then enable --now". | llama.cpp/ornith-1.5-35b-a3b |
 | v0/01-review-fixes | 2026-09-25 | done | 1 | pass | none | `Flush` now two-value. Assertion inventory (`grep -n '\.(' internal/*/*.go`): proxy.go:150 fixed to two-value; proxy_test.go:274 net/http guarantees the server writer is a Flusher. No other unchecked outside assertion. Recorder test panicked before the fix, passed after; config tests passed as-is. | llama.cpp/ornith-1.5-35b-a3b |
+| v1/03-limiter-choose | 2026-09-25 | done | 1 | pass | none | One mutex, a per-(host,model) pair with a FIFO waiter slice; release hands the slot to the head waiter by closing its channel without decrementing inflight, else frees it. A waiter whose ctx ends removes itself and, if the slot was handed in that same instant, gives it back so neither a slot nor a queue place leaks. FreeSlots counts only configured models so an unconfigured pair created by an Acquire does not add a phantom slot. | ? |
 
 ## Reviews