feat(js): m03 rules with fixtures and tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

js/src/rules.js

@@ -0,0 +1,23 @@
+import { todo } from './todo.js'
+
+/** The cell one step from `p` in direction `dir`. No wrapping: it may land outside the board. */
+export const move = (p, dir) => todo('m03')
+
+/** True if `p` lies on the board: `0 <= x < width` and `0 <= y < height`. */
+export const inBounds = (state, p) => todo('m03')
+
+/**
+ * Ask to turn the snake. Returns `state` unchanged unless the game is 'Playing' and
+ * `dir` is not the opposite of `state.dir` (the direction the snake last *moved*, not `nextDir`).
+ * Otherwise sets `nextDir = dir`. Several turns in one tick: the last allowed one wins.
+ */
+export const turn = (state, dir) => todo('m03')
+
+/**
+ * One step of the game. Returns `state` unchanged unless 'Playing'. Otherwise the head moves in `nextDir`
+ * (which becomes `dir`):
+ * - head lands on the apple: the snake grows (tail kept), score + 1, apple becomes null;
+ * - otherwise the tail cell is vacated, so the head may enter the cell the tail is leaving;
+ * - head off the board, or onto a remaining body cell: status 'GameOver', snake unchanged (but `dir` updated).
+ */
+export const advance = (state) => todo('m03')