feat: solid model of main line 4+745~5+480 (STEP + GLB), per-pattern selection, viewer rework (Task 8)

- scripts/build_solids.py: B-rep solids from standard section 03-001 (rev2, approved 2026-10-21)
  - lining per 지보패턴 run (independently selectable), excavation band per run
  - road surface (계획고, 280 mm) with per-station superelevation; base/invert ASSUMED
  - side drains (box minus pipe), H/I boxes = 공동구 (assumed); 시설한계 50 mm as reference wires only
  - staged run (4 GB RAM / 180 s per call); STEP and GLB from the same B-rep
- model/tunnel_solids.glb, model/tunnel_solids_meta.json, data/verify_solids.json
- viewer/index.html: solids, pattern and component toggles, click readout, no development text
- STEP (84.6 MB) and model/parts/ are git-ignored; the STEP is generated locally by the script

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A5Cun3RHQKna9qxE6ybZ4A

.gitignore

@@ -1,3 +1,5 @@
 data/*.tmp
 __pycache__/
 *.pyc
+model/parts/
+model/tunnel_solids.step