Add the smoke run, README and systemd unit

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

README.md

@@ -0,0 +1,106 @@
+# crossbar
+
+crossbar is an affinity router in front of several `llama-server` routers. A client's identity is
+the first path segment of its base URL; v0 routes each request to the first healthy host on that
+route's list and streams the answer back unbuffered.
+
+## Build
+
+Build everything with `make build`; the binaries land in `bin/`. Check the work with `make gate`,
+which runs the formatter, vet, tests and line-length check with no network. When the code is ready,
+run `make smoke`, which starts two fake upstreams and exercises routing, failover, recovery and
+streaming over real HTTP.
+
+## Configure
+
+crossbar reads one TOML file. This is `example.toml`:
+
+```toml
+# crossbar example configuration. Replace <tailnet> and the addresses with your own.
+listen        = "127.0.0.1:17777"   # never 0.0.0.0 — bind the tailnet address in production
+poll_interval = "1s"                # 60s in production; 1s makes the smoke run quick
+queue_max     = 8
+
+[hosts.alpha]
+base_url = "http://127.0.0.1:18081"    # e.g. http://straylight.<tailnet>:11434
+weight   = 1.0
+models   = { "ornith-1.5-35b-a3b" = { parallel = 4 }, "small-9b" = { parallel = 6 } }
+
+[hosts.beta]
+base_url = "http://127.0.0.1:18082"    # e.g. http://titan.<tailnet>:8081
+weight   = 2.0
+models   = { "ornith-1.5-35b-a3b" = { parallel = 4 } }
+
+# v0: a route is a preference list; the first healthy host that has the model wins.
+[routes.opencode-a]
+hosts         = ["alpha", "beta"]
+default_model = "ornith-1.5-35b-a3b"
+
+[routes.hermes-x]
+hosts = ["beta", "alpha"]
+```
+
+| Key | Meaning |
+| --- | --- |
+| `listen` | Where crossbar binds. A tailnet address, never `0.0.0.0`. |
+| `poll_interval` | How often each host is health-checked. 60s in production; 1s makes the smoke run quick. |
+| `queue_max` | Reserved for v1 queueing; no effect in v0. |
+| `hosts.<name>.base_url` | The llama-server base URL this host serves. |
+| `hosts.<name>.weight` | Relative share of new routes this host receives. |
+| `hosts.<name>.models` | The models this host serves, with per-model parallel tuning. |
+| `routes.<name>.hosts` | Preference order: the first healthy host that serves the model wins. |
+| `routes.<name>.default_model` | Model used when a request omits one; must be served by a host in the route. |
+
+## Run
+
+Copy the binary, the config and the unit into place, reload systemd, and start it:
+
+```sh
+install -m 0755 bin/crossbar /usr/local/bin/crossbar
+install -d -m 0755 /etc/crossbar
+install -m 0644 crossbar.toml /etc/crossbar/crossbar.toml
+install -m 0644 deploy/crossbar.service /etc/systemd/system/crossbar.service
+systemctl daemon-reload
+systemctl enable --now crossbar
+```
+
+## Point clients at it
+
+OpenCode, one provider for every project. Each instance is launched as
+`CROSSBAR_ROUTE="$(basename "$PWD")-$$" opencode`:
+
+```jsonc
+"provider": { "crossbar": { "npm": "@ai-sdk/openai-compatible",
+  "options": { "baseURL": "http://crossbar.<tailnet>:7777/{env:CROSSBAR_ROUTE}/v1" },
+  "models": { "ornith-1.5-35b-a3b": {} } } }
+```
+
+Hermes, in `config.yaml`:
+
+```yaml
+custom_providers:
+  - name: crossbar
+    base_url: http://crossbar.<tailnet>:7777/hermes-<agent>/v1
+    models: { ornith-1.5-35b-a3b: {} }
+```
+
+The route name in the URL must exist in `[routes]`; unknown routes are 404.
+
+## Inspect
+
+`GET /_crossbar/hosts` reports every host's health and loaded models:
+
+```json
+{"alpha":{"healthy":true,"loaded":["ornith-1.5-35b-a3b","small-9b"],"last_ok":"2026-09-25T09:34:18Z","last_err":""},"beta":{"healthy":true,"loaded":["ornith-1.5-35b-a3b"],"last_ok":"2026-09-25T09:34:18Z","last_err":""}}
+```
+
+`GET /_crossbar/routes` reports each route's preference order and default model:
+
+```json
+{"hermes-x":{"hosts":["beta","alpha"],"default_model":""},"opencode-a":{"hosts":["alpha","beta"],"default_model":"ornith-1.5-35b-a3b"}}
+```
+
+## What v0 does not do
+
+Leases and stickiness, SQLite, `/slots`, queueing and wake-on-LAN are out of scope for v0; see
+`PLAN.md`.