linstabpy 1.0.0rc9

Release-hardening pass before 1.0.0.

Changed

• Consolidated duplicated private helpers into shared modules: config lookup
  (cfg_get, as_bool) in utils/_solver_cfg, peak_rss_bytes() in
  utils/cost_metrics, the spatial Dirichlet helpers in
  boundary_conditions/dirichlet, the backend normalizer in
  linalg/backends, and one _matvec(operator, x, adjoint=...) in the
  solver router in place of a forward/adjoint pair. Net -142 lines with no
  behavior change; every module now imports the one canonical copy instead
  of carrying a private variant that could drift.

Fixed

• Four call sites in solvers/router/stable_sweep.py still called the
  removed _adjoint_matvec after the mat-vec merge, so PETSc-backend sweeps
  raised NameError. These paths only execute where petsc4py is installed,
  which is why a PETSc-less checkout did not catch it.
• Environment-absence tests (missing nvidia-smi, missing PETSc stack,
  missing repository .venv, empty PETSc options database) now simulate the
  absent component instead of asserting properties of the host machine. The
  full suite passes unmodified on CPU-only machines and on GPU nodes.
• The Martini export tools now locate the upstream MATLAB code through
  LINSTAB2D_ROOT only, and the one parity test that reads a raw MATLAB
  fixture bundle takes its directory from LINSTABPY_MATLAB_FIXTURES_NPZ;
  the machine-specific fallback paths are gone, and the public-provenance
  scanner now rejects those path forms.

Verification

The tagged commit passes the full test suite with zero failures on both
supported stacks: 1574 tests pass on a CPU-only environment (MKL BLAS,
MUMPS) and 1584 on a GPU environment (CUDA-enabled PETSc with STRUMPACK
plus the cuDSS backend, on an NVIDIA RTX A6000). Every remaining skip
names either an optional backend absent from that environment or a
reference file that is not distributed with the repository, and between
the two environments every backend-gated test runs at least once.

The six examples (the five Martini flows plus the Example 0 derivative
protocol) were recomputed from scratch for this release — every cached
NPZ deleted first — through ./run_all_examples.sh, and all six pass.
Against the shipped MATLAB reference bundles, the Blasius and jet
resolvent gains agree to 1.7e-11 and 3.3e-12 relative, the cavity spectra
agree to 6e-6 with eigenvalue residuals below 2e-8, and the Example 3
temporal and spatial eigenpairs satisfy their discrete problems to
residuals of 5e-9 or better. (The Example 3 parity check required a
manifest repair that landed just after the tag, in 05fc7ac and
7167080; the artifacts it verifies are the ones recomputed here.)