Add persistent MCTS export/import support

[Zyxxx-xxxyZ]

Summary

This PR adds an opt-in persistent MCTS mode that keeps search state across arbitrary root changes and can export/import the persistent MCTS state as JSON.

The key behavior is that useful ancestor-root search information can be inherited by a later descendant root, while searches performed inside a descendant subtree do not contaminate the ancestor's root-local result when switching back.

Approach

• Add persistent root copies keyed by the root position/history hash, while preserving the existing non-persistent search behavior by default.
• Store persistent primitive facts with the root key that generated them: direct leaf/NN stats on nodes, edge visits on child pointers, and pending inherited visits for future roots.
• Treat aggregate NodeStats as a materialized cache. On root switches and import, rebuild the visible subtree from only the current root's tagged primitives.
• When a real ancestor-root playout traverses a descendant, record one pending inherited visit for that descendant. When the descendant becomes root, consume those pending visits by running its own normal MCTS transition, with descendant-credit propagation disabled during catch-up.
• Make persistent root move filtering non-destructive so temporarily hidden root children keep their tagged state for future switches.
• Serialize enough primitive state to reconstruct the graph, root copies, pending visits, root position/history, NN outputs, direct stats, and edge visits.

The proof sketch and invariants are documented in docs/persistent-mcts.md.

Tests

• ./build-persistent-mcts/katago runpersistentmctstests
• ./build-persistent-mcts/katago runpersistentmctsstricttests

The strict test runs deterministic SGF root-switch alignment against standard MCTS, including export/import reload checkpoints, and a stress pass. Local run summary:

Persistent MCTS tests passed
Persistent MCTS correctness SGF case 0: tests/data/sampletest/sampletest9x9.sgf
Persistent MCTS correctness SGF case 1: tests/data/humanslbigdiff.sgf
Persistent MCTS stress SGF case: tests/data/sampletest/sampletest9x9.sgf
Persistent MCTS stress nodes=106182 requestedPlayouts=107520 bound=111631
Strict persistent MCTS tests passed