HDGL SUBSTRATE — FACE SCHEDULER
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The substrate runs continuously. Programs run as FACES: concurrent projections
of the substrate, with mathematically isolated tape memory per face.

No handoff. No separate process. The substrate IS the machine.
Faces are executed one instruction per substrate tick — genuinely interleaved.

PRIVACY
  Each face's tape occupies a different 256-byte region in RAM:
    Face 0: 0xC000..0xC0FF
    Face 1: 0xC100..0xC1FF
    Face 2: 0xC200..0xC2FF
    Face 3: 0xC300..0xC3FF
  Privacy token: phi_fold(settle_genome ^ session_coord ^ face_id)
  Different session_coord each boot (derived from RDTSC) -> different token each session.
  A face cannot read or write another face's tape: the addresses are disjoint.

USB LAYOUT
  LBA 0    : MBR
  LBA 1-20 : substrate + face scheduler (stage2, 10240 bytes)
  LBA 21   : face 0 program (512 bytes max)
  LBA 22   : face 1 program
  LBA 23   : face 2 program
  LBA 24   : face 3 program

WRITING FACE PROGRAMS
  dd if=myprog.txt of=/dev/sdX bs=512 seek=21 conv=notrunc  (face 0)
  dd if=myprog.txt of=/dev/sdX bs=512 seek=22 conv=notrunc  (face 1)
  etc.

  On the image:
  dd if=myprog.txt of=hdgl_faces.img bs=512 seek=21 conv=notrunc

KEYBOARD (works while faces run — non-blocking poll)
  h    help
  f    run_fib mode (blocking, stops faces temporarily)
  p    REPL mode
  t    temple oracle
  z    summon TempleOS (phi-bridge)
  ESC  restart substrate

VERIFIED TESTS (see build.sh)
  A: No faces — substrate ticks, z key works
  B: Face 0 = 'p' — prints 0.00000000 while substrate ticks
  C: Face 0 = phi convergence — 2.0, 1.5, 1.666... interleaved with substrate
  D: Faces 0+1 simultaneously — phi + countdown, both interleaved
  E: Keyboard 'h' key during face execution — screen changes (keyboard works)
  F: Privacy — face 0 decrements, face 1 increments, no interference (saw 5,4,6)
  G: 4 faces simultaneously — phi + countdown + Hello World + tape add + 7199 ticks

INSTRUCTION SET (same as hdgl-tc, full Turing complete)
  See hdgl-tc/INSTRUCTION_SET.txt
