feat: axis-and-section skeleton GLB and three.js viewer (Task 7-8, skeleton LOD)

build_skeleton.py sweeps the confirmed standard section (R6982 inner, 300 mm
lining, R7402 excavation, crown 6.615 m above road) along the centreline for
4+745..5+480, road = 계획고, ground = 지반고 band. Walls and invert are
approximations and recorded in meta. Verified: inner radius identity, crown
height, chord length 799.9 m for 4+700..5+500. Viewer uses three.js 0.160.0.

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

data/verify_skeleton.json

@@ -0,0 +1,11 @@
+{
+  "inner_radius_max_dev_m": 8.881784197001252e-16,
+  "lining_thickness_crown_m": 0.3,
+  "crown_above_road_m (expect 6.615 at S=0 station 5000)": 6.615,
+  "road_z_4745_m": 50.62,
+  "road_z_5480_m": 67.311,
+  "plan_chord_length_total_m (4+700..5+500)": 799.905,
+  "station_range_modelled": "4745 .. 5480 (ring step 5.0 m, 148 rings)",
+  "glb_bytes": 470416,
+  "glb_reload_geometries": 5
+}