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.

docs/CAD/ux/mockups/gen_offer_table.py

@@ -16,7 +16,12 @@ import sys
 
 HERE = os.path.dirname(os.path.abspath(__file__))
 UX = os.path.dirname(HERE)
-REPO = os.path.dirname(os.path.dirname(UX))
+# One dirname more than you would expect: this generator lives at docs/CAD/ux/mockups/,
+# not docs/ux/mockups/, since the design docs moved into the CAD subfolder (bbd1989e1e).
+# With the old count REPO resolved to docs/, so OUT pointed at docs/src/.../DesignOffer.hpp,
+# which does not exist -- and --check then diffed the real generated table against an empty
+# file and reported the whole 189-line header as a difference.
+REPO = os.path.dirname(os.path.dirname(os.path.dirname(UX)))
 ATLAS = os.path.join(UX, "tool_atlas.json")
 OUT = os.path.join(REPO, "src", "slic3r", "GUI", "CAD", "DesignOffer.hpp")