Add reference-anchored concept generation

TODO.md

@@ -404,19 +404,23 @@ facade evidence is still the 2009 CC BY frame. The 8 downloaded candidates are i
 
 **Tool plan for reference-anchored generation (plan first, implement after this section's review):**
 
-- [ ] Add `scene_concepts.py generate --reference <photo.png>`: attach a licence-cleared real photo
-      as a source image to the OpenRouter image request (image-edit/image-to-image endpoint), with
-      a prompt suffix that preserves the photographed facade geometry ("keep the building's column
-      count, spacing, band heights and sign placement exactly"). Record the reference file, its
-      licence and the model/prompt/spend in the sidecar.
-- [ ] Extend `generate_textures.generate_image()` with an optional `reference_url` (base64 data URL
-      of the photo) and a `strength` argument; keep every existing call signature unchanged and
-      gate the new path behind the reference being licence-cleared in the sidecar.
+- [x] Add `scene_concepts.py generate --reference <photo.png>`: attach a licence-cleared real photo
+      as a source image to the OpenRouter image request (multimodal chat endpoint with a base64
+      data URL), with a prompt suffix that preserves the photographed facade geometry. Record the
+      reference file, its licence and page in the `.refgen.json` provenance sidecar next to the
+      generated frame.
+- [x] Extend `generate_textures.generate_image()` with an optional `reference_path` argument
+      (base64 data URL of the photo); existing call signatures unchanged, and the path is gated by
+      `scene_concepts._reference_source` requiring a refs-board `SIDECAR.md` row (or an explicit
+      `--reference-unchecked`). Verified live on the El Capitan day concept anchored on the CC BY
+      2.0 2009 facade photo; tests in `ReferenceGateTests`.
 - [ ] Add a sidecar-driven variant pass: for each real photo, generate day, night and weathered
       variants of the same framing so the concept board stays comparable with the photo board.
-- [ ] Generate facade, street and signage concept frames for the target block
+- [-] Generate facade, street and signage concept frames for the target block
       (`scene_concepts.py generate --reference ...`), commit only the selected frames, and record
-      model, prompt, spend **and source photo** in the block sidecar.
+      model, prompt, spend **and source photo** in the block sidecar. (First frame done:
+      `hollywood_boulevard_entrance_day-v2`, anchored on the facade photo; remaining framings
+      pending.)
 - [ ] Derive palette, lighting and material targets from the real-photo board and the generated
       concepts, and record the deltas the facade kit has to close.
 - [ ] Generate signage, awning and poster texture candidates with `asset_pipeline.py`; run the