P2.1: clear matched cells (pure sx)

Add the first resolution-pipeline step to the headless board model.

- Introduce an `empty` hole sentinel on the Gem enum (ordinal 6, outside
  GEM_COUNT so the RNG/pick_gem never draw it). board_dump renders holes as
  EMPTY_CHAR ('.') via a single branch in gem_char, leaving boards without
  holes byte-identical to before (existing goldens unchanged).
- clear_cells(board, mask): set every matched cell to `.empty`, leave all
  others untouched, return the count cleared.
- clear_matches(board): detect+clear in one call; returns 0 (board unchanged)
  when there are no matches.

No gravity or refill yet (P2.2 / P2.3).

tests/clear.sx applies detect->clear to hand-crafted boards (single
horizontal/vertical runs, disjoint runs, an overlapping L/T whose shared cell
clears once, and a no-match checkerboard), snapshots before/after, and asserts
matched cells became holes, non-matched cells are unchanged, and the cleared
count is exact. Locked as tests/expected/clear.{stdout,exit}.

tests/expected/clear.exit

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

tests/expected/clear.stdout

@@ -0,0 +1,97 @@
+== clear (detect -> clear) ==
+== horizontal-3 ==
+before:
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRROGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+after:
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GO...OGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+== vertical-3 ==
+before:
+OGOGOGOG
+GOGOGOGO
+OGOGOBOG
+GOGOGBGO
+OGOGOBOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+after:
+OGOGOGOG
+GOGOGOGO
+OGOGO.OG
+GOGOG.GO
+OGOGO.OG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+== disjoint-runs ==
+before:
+OGOGOGOG
+RRROGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGYG
+GOPPPOYO
+OGOGOGYG
+GOGOGOGO
+after:
+OGOGOGOG
+...OGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOG.G
+GO...O.O
+OGOGOG.G
+GOGOGOGO
+== L-and-T ==
+before:
+OGOGOGOG
+GRRRGOGO
+OROGOGOG
+GRGOGOGO
+OGOGOGOG
+GOGYYYGO
+OGOGYGOG
+GOGOYOGO
+after:
+OGOGOGOG
+G...GOGO
+O.OGOGOG
+G.GOGOGO
+OGOGOGOG
+GOG...GO
+OGOG.GOG
+GOGO.OGO
+== no-matches ==
+before:
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+after:
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+ok: clear over hand-crafted boards