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.

cmd/imagen/main.go

@@ -18,6 +18,8 @@ const helpText = `imagen — model-agnostic image generation
 
 Usage:
   imagen generate <prompt> [flags]    generate one image
+  imagen compare <prompt> --models a,b,c [flags]
+                                       run one prompt across N backends + contact sheet
   imagen worker [flags]               consume the imagen.jobs queue (daemon)
   imagen backends                     list registered backend types
   imagen config init                  print a sample imagen.yaml on stdout
@@ -46,6 +48,8 @@ func main() {
 	switch os.Args[1] {
 	case "generate":
 		err = runGenerate(ctx, args)
+	case "compare":
+		err = runCompare(ctx, args)
 	case "worker":
 		err = runWorker(ctx, args)
 	case "backends":