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

data/alignment_plan_001_centre.csv

@@ -0,0 +1,10 @@
+station_m,centre_x,centre_y,side_gap_m
+4700,151067.261,483726.735,8.554
+4800,151119.817,483641.679,11.191
+4900,151169.128,483554.701,13.981
+5000,151215.122,483465.925,16.844
+5100,151258.471,483375.815,18.211
+5200,151301.3,483285.451,17.956
+5300,151344.993,483195.51,15.809
+5400,151390.045,483106.254,12.322
+5500,151437.284,483018.131,9.389

data/alignment_plan_002_centre.csv

@@ -0,0 +1,10 @@
+station_m,centre_x,centre_y,side_gap_m
+4700,151067.261,483726.735,8.554
+4800,151119.817,483641.679,11.191
+4900,151169.128,483554.701,13.981
+5000,151215.122,483465.925,16.844
+5100,151258.471,483375.815,18.211
+5200,151301.3,483285.451,17.956
+5300,151344.993,483195.51,15.809
+5400,151390.045,483106.254,12.322
+5500,151437.284,483018.131,9.389