Hybrid models through SBML Spatial, and vcml_convert_units

[jcschaff]

1. VCell submodule bump: PDE/particle hybrids through SBML Spatial. The submodule now points at virtualcell/vcell#2175.

• vcml_to_sbml writes spatial stochastic (Smoldyn / PDE-particle hybrid) applications. Each species carries its representation as <vcell:SpeciesContextSpecSettings vcell:representation="continuous|particle"/>; absent means continuous.
• sbml_to_vcml reads that representation back as a hybrid application.
• Merge order: merge #2175 first, then repoint the submodule at its merge commit.

2. New vcml_convert_units(vcml_content, unit_system, vcml_file_path). It rewrites a VCML BioModel in another model unit system, converting every quantity, geometry included, with VCell's own ModelUnitConverter.

• "vcell" means µm, µM and s: VCell's default units, and what its spatial and stochastic math assumes.
• "sbml" means the units VCell writes SBML in: dm, µmol, l and s.
• Why it's needed: VCell imports SBML in the SBML's own units, so a converted model can otherwise come back in dm. pyvcell's convert_units (in a companion pyvcell PR) wraps this.
• It's a new C entry point, vcmlConvertUnits, plus its ctypes binding and Python API.

Tests

• vcell-native:
  • test_hybrid_vcml_to_sbml_to_vcml: a hybrid with A as particles and B forced continuous goes VCML → SBML → VCML. The annotations come out, and so do Stochastic="true" and the ForceContinuous flags.
  • test_vcml_convert_units: TinySpatial (dm) → VCell units → SBML units, plus rejection of an unknown unit system.
• Python: test_vcml_convert_units, the same conversion through the native library.
• Changed: testSbmlToFiniteVolumeInput_not_well_formed now expects SBMLImportException("Unable to read SBML stream"). The newer vcell-core reads the SBML stream itself, so JSBML's ClassCastException no longer surfaces, which is the improvement the test's TODO asked for.
• Results: locally on macOS arm64 (GraalVM CE 23.0.2), the vcell-native tests pass (32), and so does tests/test_libvcell.py (20) against the native library from scripts/local_build_native.sh.

Context: Phase 6 of viva-pde-particle, describing hybrid models in SBML Spatial. With this build, a hybrid run on the native VCell solver from its SBML (SBML → VCell import → vcell-fvsolver) is bit-identical to the run from the original VCML with the same seed.

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