Make the ladder runnable on this fork's rig — 6 of 7 rungs now hold

This fork had never been gated end to end. Five separate things stopped it, none
of them a defect in the CAD code itself, and each failure named the wrong
subsystem — which is why they survived.

1. run-all-checks.sh invoked docs/ux/mockups/gen_offer_table.py. The design docs
   moved to docs/CAD/ (bbd1989e1e) and this path did not follow, so the rung
   failed on a missing file.
2. gen_offer_table.py then resolved REPO one dirname short, because it now sits a
   level deeper. OUT pointed at docs/src/.../DesignOffer.hpp, which does not
   exist, so --check diffed the generated table against an EMPTY file and
   reported all 189 lines as a difference.
3. DISPLAY was never passed into the container. The check scripts fall back to
   ":10", which is the other fork's rig; this one's Xvfb is :11. Symptom:
   "FATAL no app window on :10", which reads like a dead app.
4. Every absolute chrome coordinate was written in the Snapmaker fork's layout.
   This fork keeps mainline's top row (File / save / undo / redo / Calibration
   with the title), so the whole chrome sits 26 px lower. At the unshifted y the
   Design-tab click landed in the toolbar and the app stayed on the Home page,
   reported as "no sketch opened after plane click + Shift+S"; the unshifted
   Construction checkbox reported "0 construction axis". The three constants now
   derive from CHROME_DY. Canvas coordinates were never affected -- clickmm()
   computes them from live canvas geometry -- which is why dozens of geometric
   properties passed exactly on a GUI that had never been driven.
5. CON_BTN was stale for every index after 5, from the sketch epic's six new
   buttons. Fixed in both forks; see the companion commit on snaporca.

pdftocairo was also missing from the rig image (installed there, not a repo
change), without which every corpus sheet threw.

RESULT: offer-table, kernel, engine, corpus, corpus-scale and offer all hold.
The gesture ladder now reaches D2 and applies Equal length through the Constrain
toolbar, both sides landing at 99.928133658; it then fails re-entering the sketch
after commit, filed as snaporca-eoj1 with the evidence and the next measurement
to take. Kernel here is 7701 assertions / 270 cases.

scripts/CAD/check-gui-sketching.py

@@ -209,7 +209,19 @@ def leave_sketch():
 TREE_ROW0 = (300, 215)
 
 
-DESIGN_TAB = (128, 29)
+# EVERY absolute chrome coordinate below is written in the Snapmaker fork's layout and then
+# shifted by CHROME_DY, because this fork keeps mainline OrcaSlicer's top row (File / save /
+# undo / redo / Calibration, with the document title) which that fork does not have. The whole
+# chrome — tab strip, sketch toolbar, confirm button — sits 26 px lower here.
+#
+# Getting this wrong does not look like a coordinate problem. At the unshifted y the Design tab
+# click landed in that toolbar, the app stayed on the Home page, and the first rung reported
+# "no sketch opened after plane click + Shift+S"; the unshifted Construction checkbox reported
+# "0 construction axis". Both name the wrong subsystem. Canvas coordinates are immune because
+# clickmm() derives them from the live canvas geometry — only the chrome constants need this.
+CHROME_DY = int(os.environ.get("SNAPORCA_CHROME_DY", "26"))
+
+DESIGN_TAB = (128, 29 + CHROME_DY)
 
 
 def go_design():
@@ -677,7 +689,7 @@ def corner_pair(ents):
     die("no adjacent pair in what should be a rectangle")
 
 
-CONSTRUCTION_CHECKBOX = (419, 75)
+CONSTRUCTION_CHECKBOX = (419, 75 + CHROME_DY)
 
 
 def draw_line(x0, y0, x1, y1, length, angle):
@@ -794,10 +806,22 @@ def rung_extend():
 # has not constrained anything, it has only nudged it.
 
 # Constrain-mode toolbar, measured off the rig at 1920x1080 (icon centres, 42 px apart).
-CON_BTN_Y = 76
+#
+# THIS LIST MIRRORS THE cbtn() SEQUENCE IN DesignPanel.cpp AND HAS TO BE UPDATED WHEN A BUTTON
+# IS INSERTED. Positions are computed by index, so inserting a button shifts every entry after
+# it and the map silently points at the wrong icon -- a rung then applies some OTHER constraint
+# and fails with a geometric message that says nothing about buttons.
+#
+# It has already happened once, undetected: equal_radius and collinear (after "equal"), sym_v
+# and sym_h (after "symmetric") and dist_x and dist_y (after "fix") were added while this list
+# still had 14 names. Everything from index 6 on was wrong in both forks. Nothing caught it
+# because the ladder only ever clicks "perpendicular" (3) and "equal" (5), both of which sit
+# before the first insertion. The next rung to use "tangent" would have clicked "collinear".
+CON_BTN_Y = 76 + CHROME_DY
 CON_BTN = {n: (449 + 42 * i, CON_BTN_Y) for i, n in enumerate(
     ["horizontal", "vertical", "parallel", "perpendicular", "coincident", "equal",
-     "concentric", "tangent", "midpoint", "symmetric", "angle", "radius", "diameter", "fix"])}
+     "equal_radius", "collinear", "concentric", "tangent", "midpoint", "symmetric",
+     "sym_v", "sym_h", "angle", "radius", "diameter", "fix", "dist_x", "dist_y"])}
 
 
 def draw_rect_undimensioned():
@@ -826,7 +850,7 @@ def rung_dimension():
     leave_sketch()
 
 
-CONFIRM_BTN = (1751, 75)
+CONFIRM_BTN = (1751, 75 + CHROME_DY)
 
 
 def confirm_and_reopen():