BLD trainers: OP tracing/checking, FSRS drilling, BLD tab

- bld: 3x3 sticker geometry in Speffz order, lettering schemes, the
  solver's frame (default yellow top / orange front), OP tracing with
  cycle breaks and twists, and memo checking by executing the typed
  targets as OP swaps, so any valid memo passes and errors are reported
  as the pieces left wrong.
- srs: FSRS-4.5 memory model and a weighted drill picker that favours
  forgotten, hard and recently missed items. Reviews are stored one
  entity per answer so concurrent devices merge cleanly; Letter Pair
  Gym history seeds pair difficulty.
- BLD tab: pair->word (three at a time), word->pair typing, target
  drills (name the outlined sticker / find a letter's sticker on a
  scrambled 3D cube), memo trainer on official 3BLD scrambles with
  hidden recall fields, and a setup page with orientation, buffers and
  a click-to-edit lettering net.
- PuzzleView: tap-to-pick stickers, highlight outline, face cameras.
- CubeMapping between renderer stickers and NxN slots.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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