feat: downloader for Qwen-Image weights, switch-model.sh script

Two frontier tickets from the ComfyUI map (#38), both unblocked now
that their blockers (#39 model choice, #40 lazytainer research) are
resolved.

- downloader-comfyui service (docker-compose.yml) + COMFYUI_* vars
  (.env.example): fetches Qwen-Image FP8 diffusion/text-encoder/VAE
  weights from Comfy-Org/Qwen-Image_ComfyUI, same test -f guard
  pattern as the existing downloaders. Closes #42.
- scripts/switch-model.sh: swaps GPU residency between llama-server
  and comfyui via direct `docker compose stop`/`up -d`, bypassing
  lazytainer per docs/research/lazytainer-omniroute-idle-stop.md
  (its packet-threshold detector can't distinguish OmniRoute's health
  checks from real traffic, so idle-stop can't be relied on for a
  deliberate swap). llama-server-fast stays resident throughout —
  not part of this swap. Closes #43.
- scripts/update.sh: runs the new downloader profile.

docker compose config -q validated clean.

Refs #38, #42, #43

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MrnMEdzeQzqZE5soVEXPCx

.env.example

@@ -113,3 +113,12 @@ LLAMA_FAST_GPU_LAYERS=999
 # 8192 keeps this instance's KV cache negligible.
 LLAMA_FAST_CTX_SIZE=8192
 LLAMA_FAST_PARALLEL=2
+
+# --- ComfyUI diffusion model (Qwen-Image, FP8 — see docs/research/
+# image-generation-model-choice.md and issue #42) ---
+# Three files: diffusion weights, text encoder, VAE — all from the official
+# Comfy-Org FP8 split, chosen specifically because it's the only candidate
+# with a ComfyUI workflow pre-validated on this exact GPU (gfx1201/R9700).
+COMFYUI_DIFFUSION_MODEL_FILE=qwen_image_fp8_e4m3fn.safetensors
+COMFYUI_TEXT_ENCODER_FILE=qwen_2.5_vl_7b_fp8_scaled.safetensors
+COMFYUI_VAE_FILE=qwen_image_vae.safetensors