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

model/tunnel_skeleton_meta.json

@@ -0,0 +1,33 @@
+{
+  "model": "tunnel_skeleton.glb",
+  "scope": "4745~5480 (4+745~5+480)",
+  "units": "m",
+  "origin_plan_xy": [
+    151090.911,
+    483688.46
+  ],
+  "sources": {
+    "centreline": "C0060302-001 CA-ALGN-STSM (side-tick midpoint)",
+    "road_grade_B": "C0060302-002 CR-TABL-TEX2 (계획고=노면)",
+    "GL_A": "C0060302-002 CR-TABL-TEX2 (지반고)",
+    "section": "C0060303-001 CS-TABL-TEX1/2, CS-CONC-MAJR, CS-EXCV"
+  },
+  "approximations": [
+    "walls vertical down to road; invert not modelled",
+    "lining band thickness at walls is radial-consistent but not as-built",
+    "centreline and grade linearly interpolated between nodes",
+    "superelevation sign convention assumed (z += S/100 * x, x = left of tangent)"
+  ],
+  "layers": [
+    "lining_outer_R7282",
+    "lining_inner_R6982",
+    "excavation_R7402",
+    "road_surface_계획고",
+    "ground_GL_지반고"
+  ],
+  "open": [
+    "4+700~4+745 and 5+480~5+500 excluded ([미확정])",
+    "pattern zones (03-003~008) pending Task 5",
+    "lining details (04-004~009) pending Task 6"
+  ]
+}