P1.2: match detection (3+ horizontal/vertical runs)

Add a pure-sx match detector to the board model: `find_matches` walks each
row and column once in maximal same-type spans and marks every cell in a run
of length >= 3 into a `MatchMask` (a per-cell membership set mirroring
Board.cells). Overlapping shapes (L / T where a horizontal and vertical run
share a cell) collapse to the union automatically. `dump_matches` renders the
set deterministically: matched cells show their gem char, others '.'.

Detection only — no clear/collapse/refill (that is P2.1).

tests/match_detect.sx exercises hand-crafted boards (built explicitly on a
run-free checkerboard, no seeded init): a horizontal 3-run, a vertical 3-run,
multiple disjoint runs, length-4 and length-5 runs, intersecting L and T
shapes (shared cell counted once), and a no-match board. Output is locked as
tests/expected/match_detect.stdout (+ .exit) and asserts matched-cell counts.

tests/expected/match_detect.exit

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

tests/expected/match_detect.stdout

@@ -0,0 +1,109 @@
+== horizontal-3 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRROGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+........
+........
+........
+..RRR...
+........
+........
+........
+........
+== vertical-3 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOBOG
+GOGOGBGO
+OGOGOBOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+........
+........
+.....B..
+.....B..
+.....B..
+........
+........
+........
+== disjoint-runs ==
+OGOGOGOG
+RRROGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGYG
+GOPPPOYO
+OGOGOGYG
+GOGOGOGO
+--
+........
+RRR.....
+........
+........
+......Y.
+..PPP.Y.
+......Y.
+........
+== len4-and-len5 ==
+OGOGOGOG
+GRRRRRGO
+OGOGOGOB
+GOGOGOGB
+OGOGOGOB
+GOGOGOGB
+OGOGOGOG
+GOGOGOGO
+--
+........
+.RRRRR..
+.......B
+.......B
+.......B
+.......B
+........
+........
+== L-and-T ==
+OGOGOGOG
+GRRRGOGO
+OROGOGOG
+GRGOGOGO
+OGOGOGOG
+GOGYYYGO
+OGOGYGOG
+GOGOYOGO
+--
+........
+.RRR....
+.R......
+.R......
+........
+...YYY..
+....Y...
+....Y...
+== no-matches ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+........
+........
+........
+........
+........
+........
+........
+........
+ok: match detection over hand-crafted boards