feat(treetci): add DefaultTreeGlobalPivotFinder

[shinaoka]

Summary

Add global pivot search to TreeTCI optimize loop, adapted from TCI2's DefaultGlobalPivotFinder.

Design

Design spec at Tensor4all.jl/docs/specs/2026-03-30-treetci-global-pivot-design.md

Key points:

• TreeTciOptions に global_search_interval, max_global_pivots, num_global_searches, global_search_tol_margin 追加
• Greedy local search: random starting points → dimension sweep → error maximization
• to_treetn で materialization → TreeTN::evaluate で近似値 → batch_eval で真値 → 差で誤差
• global_search_interval で頻度制御 (0=disabled)
• テスト: TCI.jl parity の nasty function exp(-10x) * sin(2π * 100 * x^1.1) on chain tree

Dependencies:

• ColMajorArray ✅ (merged in refactor: API unification - ColMajorArray, IndexId evaluate, tensorci removal #379)
• TreeTN::evaluate batch ✅ (merged in refactor: API unification - ColMajorArray, IndexId evaluate, tensorci removal #379)
• crossinterpolate2 unified API ✅ (merged in refactor: API unification - ColMajorArray, IndexId evaluate, tensorci removal #379)