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

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

examples/sha256/keyboard.ne

@@ -0,0 +1,62 @@
+// sha256/keyboard.ne — key-to-character lookup + input dispatch.
+//
+// The on-screen keyboard is a 5-row × 8-column grid of cells.
+// `kb_row` and `kb_col` in state.ne store the cursor position.
+// The character produced when the user presses A is looked up
+// here. Special values are encoded with ASCII control codes so
+// they're distinguishable from printable bytes:
+//
+//     KEY_BKSP   (0x08)   — bottom row col 6. Delete one char.
+//     KEY_ENTER  (0x0A)   — bottom row col 7. Start compression.
+//
+// The grid layout (matches the keyboard in background.ne):
+//
+//     row 0:  A B C D E F G H
+//     row 1:  I J K L M N O P
+//     row 2:  Q R S T U V W X
+//     row 3:  Y Z 0 1 2 3 4 5
+//     row 4:  6 7 8 9 _ . < >
+
+var KB_CHARS: u8[40] = [
+    0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,   // A B C D E F G H
+    0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50,   // I J K L M N O P
+    0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,   // Q R S T U V W X
+    0x59, 0x5A, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35,   // Y Z 0 1 2 3 4 5
+    0x36, 0x37, 0x38, 0x39, 0x20, 0x2E, 0x08, 0x0A    // 6 7 8 9 ' ' '.' BKSP ENTER
+]
+
+// Look up the character emitted by the key at (kb_row, kb_col).
+fun current_key() -> u8 {
+    var idx: u8 = (kb_row << 3) + kb_col           // row * 8 + col
+    return KB_CHARS[idx]
+}
+
+// Append a byte to the input buffer. Does nothing if the buffer
+// is already at INPUT_MAX — users wanting to correct their
+// input reach for backspace first.
+fun input_append(ch: u8) {
+    if msg_len >= INPUT_MAX {
+        return
+    }
+    msg[msg_len] = ch
+    msg_len += 1
+}
+
+// Delete the last byte, if any.
+fun input_backspace() {
+    if msg_len == 0 {
+        return
+    }
+    msg_len -= 1
+    msg[msg_len] = 0
+}
+
+// Wipe the entire buffer. Triggered by SELECT from any state.
+fun input_clear() {
+    var i: u8 = 0
+    while i < INPUT_MAX {
+        msg[i] = 0
+        i += 1
+    }
+    msg_len = 0
+}