USB-KEYBOARD-TO-SNES BRIDGE: SUPPORTED KEYSTROKES
==================================================

Each keystroke is encoded as a single byte and transmitted in one SNES
poll frame with D-pad DOWN held (frame marker) plus the 8 face/shoulder
buttons carrying the byte's bits (LSB at B, MSB at R).

Byte ranges:
  0x00          reserved (never sent)
  0x01 - 0x7F   ASCII (printable + standard controls)
  0x80 - 0xFF   extended keys (no ASCII equivalent)


PRINTABLE ASCII
---------------

Letters (US layout, Shift composes uppercase):

  Key      Unshifted   Shifted
  ----     ---------   -------
  a        0x61  a     0x41  A
  b        0x62  b     0x42  B
  c        0x63  c     0x43  C
  d        0x64  d     0x44  D
  e        0x65  e     0x45  E
  f        0x66  f     0x46  F
  g        0x67  g     0x47  G
  h        0x68  h     0x48  H
  i        0x69  i     0x49  I
  j        0x6A  j     0x4A  J
  k        0x6B  k     0x4B  K
  l        0x6C  l     0x4C  L
  m        0x6D  m     0x4D  M
  n        0x6E  n     0x4E  N
  o        0x6F  o     0x4F  O
  p        0x70  p     0x50  P
  q        0x71  q     0x51  Q
  r        0x72  r     0x52  R
  s        0x73  s     0x53  S
  t        0x74  t     0x54  T
  u        0x75  u     0x55  U
  v        0x76  v     0x56  V
  w        0x77  w     0x57  W
  x        0x78  x     0x58  X
  y        0x79  y     0x59  Y
  z        0x7A  z     0x5A  Z

Digit row:

  Key      Unshifted   Shifted
  ----     ---------   -------
  1        0x31  1     0x21  !
  2        0x32  2     0x40  @
  3        0x33  3     0x23  #
  4        0x34  4     0x24  $
  5        0x35  5     0x25  %
  6        0x36  6     0x5E  ^
  7        0x37  7     0x26  &
  8        0x38  8     0x2A  *
  9        0x39  9     0x28  (
  0        0x30  0     0x29  )

Punctuation:

  Key      Unshifted   Shifted
  ----     ---------   -------
  -        0x2D  -     0x5F  _
  =        0x3D  =     0x2B  +
  [        0x5B  [     0x7B  {
  ]        0x5D  ]     0x7D  }
  \        0x5C  \     0x7C  |
  ;        0x3B  ;     0x3A  :
  '        0x27  '     0x22  "
  `        0x60  `     0x7E  ~
  ,        0x2C  ,     0x3C  <
  .        0x2E  .     0x3E  >
  /        0x2F  /     0x3F  ?

Space:
  Space    0x20  ' '


STANDARD CONTROL CODES
----------------------

  Key          Byte
  ----         ----
  Enter        0x0D    (CR, '\r')
  Escape       0x1B    (ESC)
  Backspace    0x08    (BS,  '\b')
  Tab          0x09    (HT,  '\t')
  Delete       0x7F    (DEL)


EXTENDED KEYS (no ASCII equivalent)
-----------------------------------

Arrow keys:

  Key           Byte
  ----          ----
  Up Arrow      0x80
  Down Arrow    0x81
  Left Arrow    0x82
  Right Arrow   0x83

Navigation:

  Key           Byte
  ----          ----
  Home          0x84
  End           0x85
  Page Up       0x86
  Page Down     0x87
  Insert        0x88
  Print Screen  0x89
  Pause / Break 0x8A

Function keys:

  Key      Byte
  ----     ----
  F1       0x90
  F2       0x91
  F3       0x92
  F4       0x93
  F5       0x94
  F6       0x95
  F7       0x96
  F8       0x97
  F9       0x98
  F10      0x99
  F11      0x9A
  F12      0x9B


UNSUPPORTED / NOT MAPPED
------------------------

These keys are recognized by the USB host but produce no byte on the
wire. Either no obvious semantic or deferred for later.

  Caps Lock          (state toggle, would need internal tracking)
  Scroll Lock
  Num Lock
  Numeric keypad     (0-9, +, -, *, /, Enter, .)
  Non-US '#' / '~'   (ISO-layout-only key)
  All modifier keys when pressed alone
    (Shift, Ctrl, Alt, GUI/Win/Cmd -- they only act as composers
     on top of other keys)


WIRE PROTOCOL NOTES
-------------------

  Idle frame:    no DOWN pressed, no buttons pressed.
  Data frame:    DOWN pressed + byte encoded in 8 buttons.
  Hold/gap:      each byte is held on the wire for ~17 ms (1 frame at
                 60 Hz) and followed by ~17 ms of idle, so consecutive
                 identical bytes are distinguishable.
  Autorepeat:    initial delay 500 ms, then ~30 bytes/sec while held.

Bit layout of the payload byte on the wire:

  payload bit    SNES button
  -----------    -----------
  0  (LSB)       B
  1              Y
  2              Select
  3              Start
  4              A
  5              X
  6              L
  7  (MSB)       R

D-pad LEFT, RIGHT, UP are reserved for future opcode/framing use.