P3.1: base match scoring by run length (pure sx)

Add a run-enumeration path and base scoring to the pure-sx model, scoring
one resolution round's clears purely by maximal-run length.

- find_runs(board) enumerates each maximal H/V run (len >= 3) with its
  length, parallel to find_matches/MatchMask (left untouched so every
  clear/cascade caller is unaffected).
- run_score(len): length 3 -> 30, 4 -> 60, 5+ -> 100 (named constants).
- score_round(board): sum of run_score over the current runs (read-only,
  must be called before the round's clear). L/T rule: each maximal run
  scores independently by its own length, so an overlapping L/T scores
  horizontal + vertical (shared corner counts toward both runs).
- Board gains a running `score` field (init zeroes it) and add_round_score
  accumulates a round's base points into it; the cross-round combo
  multiplier off Cascade.depth is left for P3.2.
- tests/score.sx golden over hand-crafted single-round boards asserts exact
  points for len-3/4/5 runs, disjoint runs (sum), an overlapping L/T, and a
  no-match board (0).

tests/expected/score.exit

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

tests/expected/score.stdout

@@ -0,0 +1,84 @@
+== score (base match scoring) ==
+== len3-run-30 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRROGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+1 runs
+H len 3 at fixed 3 start 2
+points 30
+== len4-run-60 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRRRGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+1 runs
+H len 4 at fixed 3 start 2
+points 60
+== len5-run-100 ==
+OGOGOGOG
+GRRRRRGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+1 runs
+H len 5 at fixed 1 start 1
+points 100
+== disjoint-30+60+100 ==
+RRRGOGOG
+GOGOGOGO
+GOBBBBOG
+OGOGOGOG
+GPPPPPGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+--
+3 runs
+H len 3 at fixed 0 start 0
+H len 4 at fixed 2 start 2
+H len 5 at fixed 4 start 1
+points 190
+== overlap-LT-per-run-120 ==
+OGOGOGOG
+GRRRGOGO
+OROGOGOG
+GRGOGOGO
+OGOGOGOG
+GOGYYYGO
+OGOGYGOG
+GOGOYOGO
+--
+4 runs
+H len 3 at fixed 1 start 1
+H len 3 at fixed 5 start 3
+V len 3 at fixed 1 start 1
+V len 3 at fixed 4 start 5
+points 120
+== no-match-0 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+--
+0 runs
+points 0
+ok: base scoring over hand-crafted boards