MPC export: fit inside a safety-margin inset, not just the bare trim

[unforcedagi]

Summary

• MakePlayingCards flagged an "uneven borders" preflight warning on order 8C4D0D6B92857129BF1D3C3FC6735398 and put it on hold.
• Root cause: our art is a true 2:3 ratio, narrower than MPC's 3.5x5" jumbo trim (0.70 ratio). The export fit art by height to the trim and padded only the leftover width to reach it — dumping the whole aspect-ratio shortfall into the horizontal margin, leaving vertical margin under MPC's own documented 1/8" safety-margin requirement (a second allowance inside the trim line, beyond bleed) on most cards once trimmed.
• Fix: fit into a box already inset by that safety margin on every side, then fill out to the full upload canvas with the same edge-stretch bleed print_prep.py already uses safely. Works regardless of how close any given card's own border sits to its own edge (varies a lot between the Shell realm's ornate frame and the thinner Roots/Trunk/Branches line).

Test plan

• Regenerated all 63 fronts + back via python3 tools/export_mpc.py — no errors, no missed corner marks.
• Verified by simulating the actual trim cut and measuring where the flat stretched fill ends and real texture begins (not just "first dark pixel," which false-flagged cards whose border is black at their own true edge) — 55/63 cards clear MPC's 37px safety threshold on all four sides outright, the remaining 8 are within 1-2px (rounding, not structural).
• Visually spot-checked both border styles (Shell realm's tight-margin ornate frame, Roots/Trunk/Branches' wider tan-margin line) plus a title/extra card — titles, rank medallions, and corner glyphs all composited correctly at the new canvas size.
• Aaron to eyeball the two sample renders and approve before re-upload to the live MPC order.