Add heuristic option with git-style cost cap

[krkarma777]

Summary

• New DiffOptions.heuristic (default false = exact): caps each subproblem's middle-snake search at max(64, sqrt(N+M)) phases; past the cap it splits at the furthest-reaching point of either direction (the git xdiff strategy)
• Measured on the 8 KB completely-different char worst case: 217 ms → 8.0 ms (27×) with an edit script only +8.1% above minimal — versus diff-match-patch's own heuristic mode at ~657 ms on the same input
• While the true edit distance is under the cap, output is byte-identical to exact mode (tested); round-trip validity always holds
• diffTokens gains an optional options argument carrying the flag; refine sub-diffs inherit it

Test Plan

• 5 new tests: exact-equivalence under the cap (600 pairs), round-trip fuzz on wildly different inputs (450 diffs), edit-count sanity vs exact, refine interplay, trivial cases
• Full suite 39/39, typecheck clean, build OK