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/workflows/sd35-medium.json

@@ -0,0 +1,66 @@
+{
+  "4": {
+    "class_type": "CheckpointLoaderSimple",
+    "inputs": {
+      "ckpt_name": "${model}"
+    }
+  },
+  "6": {
+    "class_type": "CLIPTextEncode",
+    "inputs": {
+      "text": "${prompt}",
+      "clip": ["4", 1]
+    }
+  },
+  "7": {
+    "class_type": "CLIPTextEncode",
+    "inputs": {
+      "text": "${negative}",
+      "clip": ["4", 1]
+    }
+  },
+  "8": {
+    "class_type": "VAEDecode",
+    "inputs": {
+      "samples": ["31", 0],
+      "vae": ["4", 2]
+    }
+  },
+  "9": {
+    "class_type": "SaveImage",
+    "inputs": {
+      "filename_prefix": "imagen",
+      "images": ["8", 0]
+    }
+  },
+  "13": {
+    "class_type": "ModelSamplingSD3",
+    "inputs": {
+      "model": ["4", 0],
+      "shift": "${shift}"
+    }
+  },
+  "27": {
+    "class_type": "EmptySD3LatentImage",
+    "inputs": {
+      "width": "${width}",
+      "height": "${height}",
+      "batch_size": 1
+    }
+  },
+  "31": {
+    "class_type": "KSampler",
+    "inputs": {
+      "model": ["13", 0],
+      "seed": "${seed}",
+      "steps": "${steps}",
+      "cfg": "${cfg}",
+      "sampler_name": "${sampler}",
+      "scheduler": "${scheduler}",
+      "denoise": 1.0,
+      "positive": ["6", 0],
+      "negative": ["7", 0],
+      "latent_image": ["27", 0]
+    }
+  }
+}