mAi: #10 - multi-model backend expansion (workflow templates + compare harness)

Path 1 architecture: one comfyui adapter, workflows as data.

- workflow_template.go: embed.FS + token substitution with type-preserving
  whole-value placeholders. ${prompt} → string, ${seed} → int64,
  ${cfg} → float64 — no JSON round-tripping. Partial matches ignored.
- comfyui.go: refactored to load workflow from embedded FS or filesystem
  path. Back-compat preserved: workflow: defaults to flux1-schnell.
- workflows/{flux1-schnell,flux2-klein,sd35-medium}.json — bundled
  templates. flux1-schnell migrated from hardcoded with identical node IDs.
- compare.go: new `imagen compare` subcommand. Sequential N-backend run
  (one GPU on mRock — parallel would OOM), per-backend PNG, sidecar JSON
  with per-model metadata + errors, composite contact sheet via Go image
  package (no ImageMagick dep).
- Sample config gains flux2-klein-local + sd35-medium-local instances.
- docs/backends.md: architecture rationale + per-model HF download paths
  + how to add a new bundled workflow + compare-harness reference.

Live smoke verified: compare mock + flux-schnell-local at 768×768 →
both PNGs written, sidecar JSON has workflow="flux1-schnell" + full
metadata, contact sheet renders. Worker contract (Request → Generate)
unchanged, so flexsiebels /imagine UI API surface preserved.

Tests: 11 existing comfyui + 6 new workflow_template + 5 new compare
tests, all green.

Adding a new model is now yaml + JSON, never Go.

internal/backend/comfyui_test.go

@@ -312,7 +312,7 @@ func TestComfyMissingModelHintsAtSetupDoc(t *testing.T) {
 		t.Fatal("expected error")
 	}
 	msg := err.Error()
-	if !strings.Contains(msg, "docs/setup-comfyui-mrock.md") {
+	if !strings.Contains(msg, "docs/backends.md") {
 		t.Errorf("error should point at the setup doc, got %v", err)
 	}
 	if !strings.Contains(msg, "flux1-schnell.safetensors") {
@@ -331,7 +331,7 @@ func TestComfyMissingModelOn200WithNodeErrors(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error for node_errors on 200")
 	}
-	if !strings.Contains(err.Error(), "docs/setup-comfyui-mrock.md") {
+	if !strings.Contains(err.Error(), "docs/backends.md") {
 		t.Errorf("error should point at the setup doc, got %v", err)
 	}
 }