mAi: #2 - phase 2 ComfyUI Go adapter, tests, config sample

internal/backend/comfyui.go implements the Backend interface against
ComfyUI's /prompt + /history + /view HTTP API. Workflow is the canonical
FLUX.1 schnell shape — UNETLoader + DualCLIPLoader (clip_l + t5xxl fp8) +
VAELoader + ModelSamplingFlux + KSampler — assembled as a Go map per
request so Width / Height / Seed / Steps / sampler / scheduler all flow
into the right node inputs.

Resilience: one retry on /prompt 5xx and transient network errors, no
retry on 4xx. Connection-refused / timeouts surface a 'boot-whitetower
mrock' hint. node_errors mentioning a missing unet point users at
docs/setup-comfyui-mrock.md (matches both the 4xx and 200-with-errors
shapes ComfyUI uses across versions).

Result.Metadata carries model, seed_used, latency_ms, steps, sampler,
scheduler, width, height, prompt_id, client_id, plus best-effort
vram_used_mib pulled from /system_stats post-gen.

Tests use httptest with poll interval squashed to 1ms — no real mRock
dependency. Coverage: happy path, defaults, retry-once on 5xx, give-up
after two 5xx, no-retry on 4xx, missing-model hint (both 4xx and
200+node_errors paths), history-error surfaced, /view 4xx, unreachable
host, ctx cancel during poll, workflow-shape assertion, registration.

Config sample: flux-schnell-local is now default_backend; the user-facing
block names the unet file by basename (the mapping into models/unet/ is
the server's convention, captured in docs/setup-comfyui-mrock.md from
phase 1).

Smoke verified end-to-end: imagen generate ... --backend
flux-schnell-local --size 1024x1024 --output /tmp/cat-via-cli.png on
mRock returned a 1024x1024 PNG of a cat in a fishbowl in 10.3s with a
sidecar carrying seed + latency_ms + the rest of the metadata.

internal/config/config_test.go

@@ -16,7 +16,7 @@ func TestLoadAndValidate(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Load: %v", err)
 	}
-	if cfg.DefaultBackend != "mock" {
+	if cfg.DefaultBackend != "flux-schnell-local" {
 		t.Errorf("default = %q", cfg.DefaultBackend)
 	}
 	mock, ok := cfg.Backends["mock"]
@@ -30,9 +30,15 @@ func TestLoadAndValidate(t *testing.T) {
 	if !ok {
 		t.Fatalf("flux backend missing")
 	}
+	if flux.Type != "comfyui" {
+		t.Errorf("flux type = %q", flux.Type)
+	}
 	if flux.Raw["base_url"] != "http://mrock:8188" {
 		t.Errorf("flux base_url = %v", flux.Raw["base_url"])
 	}
+	if flux.Raw["model"] != "flux1-schnell.safetensors" {
+		t.Errorf("flux model = %v", flux.Raw["model"])
+	}
 }
 
 func TestValidateRejectsUnknownDefault(t *testing.T) {