fix: VTK target reuse, OxidationStrategy refactor

[FilipovicLado]

• VTK double-loading fix: removes the ViennaPS-side workaround that
  prevented ViennaLS from double-loading VTK when consumed as a
  subdirectory. The fix now lives in ViennaLS ≥5.8.3, which checks for
  existing VTK::CommonCore targets before calling find_package(VTK).
  Bumps ViennaLS dependency floor to >=5.8.3.

• OxidationStrategy: introduces a dedicated process strategy for
  models that manage their own time-stepping physics. Models override
  managesOwnPhysics() and applyModel() on ProcessModelBase;
  OxidationStrategy dispatches to them directly, replacing the
  previous AdvectionCallback/CallbackOnlyStrategy indirection.
  Extensible to future self-contained models (anneal, implant diffusion,
  etc.) without changes to psProcess.

• psOxidation refactor: removes the OxidationCallback inner class,
  deduplicates the three material-search loops via a MaterialIndices
  helper, renames doApplyPreAdvect → run, resets
  toleranceWarningEmitted_ when tolerances are changed between runs,
  and removes a dead maxVelocity default parameter.

• macOS arm64 CI: pins the package runner to macos-15 —
  macos-latest now resolves to macOS 26 (Tahoe) and breaks wheel
  builds because Homebrew libs are compiled with a 26.0 minimum
  deployment target.

Tests

• All CI checks pass (Linux, Windows, macOS x86_64 + arm64)
• All oxidation examples run correctly end-to-end
• Python bindings build and import on all platforms