Add forward plane motion phases

[sabino]

Summary

• add separate floor/ceiling forward phase knobs for the baked plane cache, defaulting simple-map builds to 2 phases each
• bake plane tiles in direction -> forward phase -> row -> column order and drive runtime phase from player position projected onto view direction
• include direction, lateral scroll, and both forward phases in the backdrop cache key without touching wall invalidation or movement speed
• add a plane-motion screenshot checker and docs for the new knobs/validation flow

Validation

• python3 -m py_compile tools/*.py
• git diff --check
• make cart DOOM_IWAD=/home/sabino/Downloads/Doom1.WAD BUILDDIR=build/plane-motion ROM=build/plane-motion-rom GFX_ROM_DIR=build/plane-motion-assets
• make key-test-rom DOOM_IWAD=/home/sabino/Downloads/Doom1.WAD
• make key-door-test-rom DOOM_IWAD=/home/sabino/Downloads/Doom1.WAD
• make combat-test-rom DOOM_IWAD=/home/sabino/Downloads/Doom1.WAD
• launched build/plane-motion-rom in visible GnGeo for manual feel check

Not run before opening: movement bench, powerup-test-rom.