fix(combat): Thread 5-10-5 diagonal parity through all budget-consuming paths

The greedy move/run/flee/charge descent helpers hardcoded diagonal parity 0,
letting a speed-30 walker cover 6 diagonal cells (cell-count budget). Under
5-10-5 (SRD Movement, Tactical) the same walker covers only 4 cells
(1+2+1+2=6 squares).

Threading the running diagonal count:
- _best_move_step / _best_flee_step: accept diagonals_used param
- _descend_path / _run_line_path / _flee_path: track and pass parity
- _charge_end: use _path_cost_5105() instead of len(path) for max check
- _path_cost_5105: new helper; computes total 5-10-5 cost of any path

Updated golden paths in test_aoo_limit_one_per_round (5-cell path now
takes an orthogonal detour to avoid expensive 2nd diagonal), test_run_*
 (speed-30 diagonal move stops at 4 cells), and staggered_withdraw
(single-speed diagonal withdraw stops at 4 cells).

tests/test_combat.py

@@ -1071,7 +1071,10 @@ def test_aoo_limit_one_per_round() -> None:
         "initiative: guard d20=9+0=9",
         "initiative: dest d20=8+0=8",
         "round 1 guard: AoO vs runner d20=15+5=20 AC 12 -> HIT 3 damage (20->17)",
-        "round 1 runner: move (0,0)->(0,1)->(1,2)->(2,3)->(3,3)",
+        # 5-10-5 parity: the second diagonal (1,2)->(2,3) now costs 2, so the
+        # greedy walk prefers the orthogonal (1,2)->(1,3) detour (both paths
+        # total 5 squares; the new one avoids the expensive second diagonal).
+        "round 1 runner: move (0,0)->(0,1)->(1,2)->(1,3)->(2,3)->(3,3)",
         "round 1 guard: wait",
         "round 1 dest: wait",
         "battle over: draw after 1 rounds",