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examples/sha256: interactive SHA-256 hasher with on-screen keyboard

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An end-to-end FIPS 180-4 SHA-256 hasher running entirely on the NES.
The player types up to 16 ASCII characters on a 5x8 on-screen
keyboard, presses Enter, and the program computes and displays the
64-character hex digest.

Layout (`examples/sha256/*.ne`):
  constants.ne         layout + K[64] / H_INIT[8] tables
                       (declared as `var` with init_array because the
                       v0.1 compiler treats `const u8[N] = [...]` as
                       a no-op — noted in the file)
  assets.ne            44-tile Tileset (A..Z, 0..9, punctuation,
                       special keys, cursor) shared between BG and
                       sprite layers
  background.ne        static nametable (title, labels, keyboard
                       grid) painted at reset
  state.ne             globals
  sha_core.ne          32-bit byte primitives (copy, xor, and, add,
                       not, rotr, shr) in inline asm + sigma/Sigma
                       mixers + schedule/round steps + fold
  render.ne            OAM helpers for cursor, input buffer, and
                       64-nibble digest
  keyboard.ne          key dispatch table
  entering_state.ne    cursor navigation + typing + auto-demo
  computing_state.ne   phased driver (48 schedule steps + 64 rounds
                       + fold across ~30 frames at 4 iterations each)
  showing_state.ne     renders the 256-bit digest as 8 rows of 8
                       sprite glyphs

Implementation notes:
  - All 32-bit words live as 4 little-endian bytes in `wk[64]`,
    `w[256]`, `h_state[32]` so every primitive walks four bytes with
    `LDA {arr},X`/`STA {arr},X` chains and, for adds, a carry chain.
  - Every primitive reads its parameters straight out of the
    transport slots `$04`/`$05` rather than `{dst}`/`{src}`
    substitutions: the inline-asm resolver looks parameters up in
    the analyzer's allocation table but the codegen spills them to a
    different per-function RAM slot, so `{dst}` would resolve to a
    ZP slot nothing ever writes to. Bypassing the substitution
    entirely sidesteps the issue without a compiler change.
  - Rotate-right by any amount is a byte-rotate loop plus a bit-
    rotate loop so the 10 SHA amounts (2, 6, 7, 11, 13, 17, 18, 19,
    22, 25) all compile to a handful of chained `ROR`s.
  - The headless jsnes golden auto-types "NES" after 1 s of idle and
    captures its SHA-256 digest
    AE9145DB5CABC41FE34B54E34AF8881F462362EA20FD8F861B26532FFBB84E0D
    — byte-identical to `shasum` / `hashlib.sha256(b"NES")`.

Build: `cargo run --release -- build examples/sha256.ne`

https://claude.ai/code/session_01FRmSBruVWCufm3LsUVMs8v
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// sha256/background.ne — the static nametable.
//
// Painted once at reset-time (the first `background` declaration
// in the program is loaded before rendering is enabled). Contains
// the title banner, section labels, and the 5 × 8 keyboard grid.
// The Entering and Showing state handlers draw the dynamic input
// buffer, the cursor, and the 64-digit hash on top of this
// background via sprites.
//
// Legend characters are chosen to read cleanly when glanced over
// as ASCII art:
// upper-case A-Z and 0-9 → their matching glyph tile
// space ' ' → blank tile (tile 44)
// ':' ',' '-' '.' → punctuation glyph tiles
// '_' → space-key glyph on the keyboard
// 'p' → period-key glyph on the keyboard
// '<' '>' → backspace / enter key glyphs
//
// `palette_map:` leaves every metatile on bg sub-palette 0; the
// keyboard keys render as lt_gray (palette index 2), which reads
// cleanly against the dk_blue universal background.
background Screen {
legend {
" ": 44 // blank (universal colour)
"A": 1
"B": 2
"C": 3
"D": 4
"E": 5
"F": 6
"G": 7
"H": 8
"I": 9
"J": 10
"K": 11
"L": 12
"M": 13
"N": 14
"O": 15
"P": 16
"Q": 17
"R": 18
"S": 19
"T": 20
"U": 21
"V": 22
"W": 23
"X": 24
"Y": 25
"Z": 26
"0": 27
"1": 28
"2": 29
"3": 30
"4": 31
"5": 32
"6": 33
"7": 34
"8": 35
"9": 36
"_": 37 // space-key glyph (underscore bar)
"p": 38 // period-key glyph
":": 39
"-": 40
"<": 41 // backspace
">": 42 // enter
}
map: [
" ", // row 0
" SHA-256 HASHER ", // row 1 — title
" ", // row 2
" INPUT: ", // row 3 — input label
" ", // row 4 — input row A
" ", // row 5 — input row B
" ", // row 6
" TYPE A MESSAGE PRESS > ", // row 7 — prompt
" ", // row 8
" ", // row 9
" ", // row 10
" ", // row 11
" A B C D E F G H ", // row 12 — kb row 0
" I J K L M N O P ", // row 13 — kb row 1
" Q R S T U V W X ", // row 14 — kb row 2
" Y Z 0 1 2 3 4 5 ", // row 15 — kb row 3
" 6 7 8 9 _ p < > ", // row 16 — kb row 4
" ", // row 17
" ", // row 18
" ", // row 19
" SHA-256: ", // row 20 — hash label
" ", // row 21 — hash rows
" ", // row 22
" ", // row 23
" ", // row 24
" ", // row 25
" ", // row 26
" ", // row 27
" ", // row 28
" " // row 29
]
}