feat(inner): add GlobalNystrom preconditioner (sketch once, mask per free block)

[mstoll1602]

Summary

• Adds GlobalNystrom, a new InnerSolver that sketches the full operator A once and reuses that basis across every free block the active-set loop visits, instead of resketching A[F, F] from scratch on every outer step (what Nystrom does). Restricting a rank-r factorisation to a principal submatrix is exact, so masking rows of the one global basis is a valid free-block preconditioner — applied via the general Sherman-Morrison-Woodbury identity, since the masked basis is no longer orthonormal.
• Wires it in as the "global_nystrom" string shortcut alongside cg/jacobi/nystrom/exact, and exports it from the package root.
• The sketch is memoised on operator identity (mirroring Exact's conditioning memo), so a solve with many outer steps — or several solves reusing the same operator (parameter sweeps, warm-started re-solves) — pays the expensive sketch (matrix-free products, QR, Cholesky, SVD) once instead of once per step/solve.
• Adds a marimo notebook (book/marimo/notebooks/04_solver_comparison.py) comparing CG, Jacobi, Nystrom, and GlobalNystrom head to head on a planted-optimum problem with a spectral gap, including a repeated-solve experiment showing where GlobalNystrom's pay-once sketch pulls ahead.

Test plan

• make fmt — lint/format clean
• make typecheck — ty + mypy strict clean
• make test — 112 passed, 100% coverage maintained
• make deptry — no dependency issues
• New tests: preconditioner-level correctness/rejection tests (tests/test_nncg/test_inner.py), solver-level planted-optimum + trajectory-equivalence + sketch-memoisation tests for both the bound-only and equality-constrained solves (tests/test_nncg/test_solver.py), and the "global_nystrom" API shortcut (tests/test_nncg/test_api.py)
• Notebook verified end-to-end via marimo export html (all four solvers report converged: True, both figures render)

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