instrument: TT lookup/hit counters (hot-path) + env-gated summary

[wopdevries]

Minimal, low-risk instrumentation to measure the transposition-table hit rate on the search hot path.

Changes:

• Add tt_lookup_count and tt_hit_count counters to ThreadData
• Increment counters in apply_ab_tt_lookup (the canonical AB hot-path TT probe)
• Print env-gated summary at SOLVER_DONE when DDS_PRINT_TT_STATS=1

No behavioral changes — counters are cheap increments, output is gated.

Measurement results on hands/largest.txt (21 hardest hands, single-threaded):

• TT hit rate: ~66-67% consistently across all hands
• Harder hands show slightly lower hit rate (63.67% for the hardest)
• ~1 in 3 lookups misses, suggesting TT size is the binding constraint

How to test:

bazelisk build //library/tests:dtest
DDS_PRINT_TT_STATS=1 ./bazel-bin/library/tests/dtest -f hands/largest.txt -s solve -n 1