P2.3: seeded refill (pure sx)

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).

tests/expected/refill.exit

@@ -0,0 +1 @@
+0

tests/expected/refill.stdout

@@ -0,0 +1,39 @@
+== refill (seeded) ==
+start:
+OGOGOGOG
+RRROGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGYG
+GOPPPOYO
+OGOGOGYG
+GOGOGOGO
+after clear:
+OGOGOGOG
+...OGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOG.G
+GO...O.O
+OGOGOG.G
+GOGOGOGO
+after collapse:
+.....G.G
+OG.GOO.O
+OGOOGG.G
+GOOGOOOO
+OGGOGGGG
+GOOGOOOO
+OGOGOGGG
+GOGOGOGO
+after refill:
+RROPYGGG
+OGRGOOGO
+OGOOGGPG
+GOOGOOOO
+OGGOGGGG
+GOOGOOOO
+OGOGOGGG
+GOGOGOGO
+filled 9 holes
+ok: refill fills every hole from the seeded stream