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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