Add refill to the board model: every .empty hole is filled with a fresh gem drawn from the board's OWN seeded RNG, so refills are fully reproducible for a seed and continue the stream rather than reseeding. - Board now owns its RNG state (rng: Rng); init seeds and draws from it, so draws after init/clears thread deterministically. The init draw sequence is unchanged, so board_init's golden is byte-identical. - refill(board) fills all holes in row-major order wherever they sit (does not assume collapse ran) and makes no attempt to avoid matches — a refill may create new runs, which drives the P2.4 cascade. - tests/refill.sx (fixed seed) runs clear -> collapse -> refill, locks the staged dump as a golden, and asserts: zero empties after refill; each hole holds the next seeded-stream gem (replayed from the pre-refill state); drawn gems vary (not a constant); same start+seed -> identical board; a second refill of the same holes draws new gems (RNG threads, no reseed).
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