DKX 2.6.0

Acceptance tightened where it could pass a wrong answer, MUMPS as an explicit
direct backend, and convergence and optimization reports that no longer claim
more than their evidence supports.

Correctness

• Admit each right-hand side against its own norm (#265). A multi-column solve
  admitted every column against the largest right-hand side's norm, so with
  norms [1, 1e6], residuals [1e-5, 0] and a tolerance of 1e-10 the small
  column passed while missing its own target by a factor of 100,000. Each
  column now meets max(atol, tol * ||b_j||), reused factors included; zero
  columns use the requested absolute tolerance, and nonfinite inputs, negative
  residual norms and invalid tolerances are refused.
• Include the magnetic drift's upwind support in the sparse assembly (#268).
  Streaming uses centred derivatives and the tangential magnetic drift separate
  upwind ones, whose toroidal radius is three points rather than two on the
  public reduced W7-X case; the pattern and the column groups now use the union
  of the active supports. The assembly's own 1e-10 check against the operator
  refused the old pattern, so no wrong matrix was ever factored; decks without
  magnetic drifts assemble exactly as before.
• Reject invalid optimization evidence and require a genuine refinement before
  promotion (#264): nonfinite or negative residuals, a single baseline grid
  labelled as a ladder, and a refinement that changes radius or species count
  are refused, and an absent backend comparison is reported as untested.

Execution

• Select MUMPS explicitly as the sparse direct backend,
  SolverOptions(method="direct", direct_backend="mumps", memory_budget_gb=...)
  (#267). SuperLU stays the default and the default path is unchanged. The
  budget is checked after assembly and before factorization, against measured
  process memory, the COO storage, the right-hand-side buffers and the host's
  currently available memory, and again before a retained factorization is
  applied to a wider right-hand side. An explicit MUMPS request that cannot be
  met is refused and never silently falls back to SuperLU. It needs the MUMPS
  adapter of SOLVAX 0.25.0 and PyMUMPS; with SOLVAX 0.24 it raises an
  ImportError naming what is missing.

Reporting

• Report grid convergence and original-equation acceptance as separate
  verdicts (#269). dkx converge keeps converged for the observable changes
  and adds original_equations_accepted, which requires complete, finite,
  typed residual evidence within the requested tolerance on every rung; the
  exit status is zero only when both pass. Native runs reapply the operator to
  the returned state and reject a false solver success.
• Compare bootstrap current in dkx converge, keep earlier rungs when a later
  one fails, and record failed and refused refinements without claiming
  convergence (#266). The NCSX record gains an executable reconstruction from
  pinned public sources with four SHA-256 checks.
• Report memory in one unit (#271). Traces mixed decimal MB with MiB and could
  present device capacity or a historical peak as current usage; profiling now
  uses MiB throughout and keeps current RSS, peak RSS and current device usage
  distinct.

Development

• Let a cancelled CI run stop before the final gate (#270).

Known limitation

• The sparse direct route's transposed solve can stall above its tolerance for
  a general cotangent. On the operator of
  tests/test_operator_assembly.py::DECK (1,204 unknowns) with the cotangent
  numpy.random.default_rng(0).standard_normal(1204), it reaches a relative
  residual of 3.18e-8 against the forward solve's 1e-10 on the same
  factors, in 2.5.0 and here alike; further refinement sweeps do not move it.
  The route reports converged=False rather than a value, so this is an
  accuracy limit that is reported, not a wrong answer. The 1,962-unknown
  full-FP grid of #265 reaches 2e-13, so the limit depends on the operator.