feat: draft main-line centreline stations from plan sheets (Task 3, HITL)

extract_alignment_plan.py takes the midpoint of each station's two side
ticks (CA-ALGN-STSM) as the centreline point. Sheet 001 gives 4+700..5+500
with consecutive residual <= 0.02 m. Sheet 002 gives identical output, so
the duplicate question is left to HITL. Profile EL is not yet extracted.

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

scripts/explore_alignment.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+"""Task 3 exploration: layer usage and long polylines / text clusters in a
+plan-profile DXF, to locate the tunnel centreline (본선 축선) candidates."""
+import collections
+import json
+import sys
+
+import ezdxf
+
+path = sys.argv[1]
+doc = ezdxf.readfile(path)
+msp = doc.modelspace()
+by_layer = collections.Counter(e.dxf.layer for e in msp)
+print("layers used:", len(by_layer))
+for name, n in by_layer.most_common(25):
+    print(f"  {n:6d}  {name}")
+# long polylines = candidate alignment / outline strings
+polys = []
+for e in msp:
+    if e.dxftype() == "LWPOLYLINE":
+        pts = list(e.get_points("xy"))
+        if len(pts) >= 20:
+            xs = [p[0] for p in pts]; ys = [p[1] for p in pts]
+            polys.append({"layer": e.dxf.layer, "n": len(pts),
+                          "x": [round(min(xs), 1), round(max(xs), 1)],
+                          "y": [round(min(ys), 1), round(max(ys), 1)]})
+polys.sort(key=lambda d: -d["n"])
+print("long LWPOLYLINE (>=20 vertices):", len(polys))
+for p in polys[:12]:
+    print("  ", p)
+# text that looks like station (STA) or level (EL) labels
+import re
+pat = re.compile(r"(STA|No\.?|EL|E\.L|\d+\+\d+|\+\d{2,3}\.\d{3}|\d{2,3}\.\d{2,3})", re.I)
+hits = []
+for e in msp:
+    if e.dxftype() in ("TEXT", "MTEXT"):
+        s = e.dxf.text if e.dxftype() == "TEXT" else e.plain_text()
+        if s and pat.search(s):
+            loc = e.dxf.insert if e.dxftype() == "TEXT" else e.dxf.insert
+            hits.append((round(loc.x, 1), round(loc.y, 1), s.strip()[:40]))
+print("station/level-like texts:", len(hits))
+for h in sorted(hits)[:25]:
+    print("  ", h)