MCST/AREPO: correct code_time, register defaults, silence false mismatches

[cbyrohl]

Summary

Triaging a user-reported "missing units" bug on MCST simulations at MPCDF surfaced a small cluster of independently-actionable defects in stock scida defaults. All fixes verified end-to-end on the MCST_ST8 (z=3) and MCST_ST15 (z=5.5) test snapshots: 0 non-success unit states across snapshot + groupcat for both variants.

Changes

units/gadget_cosmological.yaml

• code_time: 0.978 * Gyr -> kpc / (km/s). pint's Gyr is not Julian-year x 1e9 here, so the previous form evaluated to 3.11474e+16 s instead of the AREPO-derived 3.085678e+16 s -- 0.94% high, squaring to 1.89% on anything quadratic in code_time (energy, pressure). This surfaced as BH_CumEgyInjection_QM/RM mismatches.
• IonisingPhotonRate1e49: override: true. Some MCST AREPO builds write the to_cgs attr as the reciprocal (1e-49 instead of 1e+49) -- a sign-inversion bug in the writer's dimensional path for fields whose name encodes the scale. The unit file is authoritative.
• BH_Mdot{,Bondi,Eddington}: override: true. AREPO writes dimensionally inverted scaling attrs (length_scaling=1, mass_scaling=-1, velocity_scaling=1 -> L^2/(M*T)) while to_cgs carries the correct M/T magnitude. Trust the unit file's dimensions.
• Add MCST per-subhalo Subhalo{H2,H,He}Mass entries.

interfaces/mixins/units.py

• check_unit_mismatch now treats 'none' in the unit file as equivalent to a dimensionless metadata Quantity with cgs factor 1. Eliminates a false-positive class for integer ID / counter fields (StromgrenSourceID, NumberOfSupernovae, ...) whose writer emits to_cgs=1 and zero scalings.

configfiles/config.yaml

• Add /vera/ptmp/gc/MCST to stock datafolders, alongside the other MPCDF paths. MCST runs are loadable by run-name without per-user config.

tests/testdata.yaml

• Register MCST_ST15_{snapshot,group}_z5p5 next to the existing ST8 z=3 entries, picking up the new z=5.5 variant already on disk.

Test plan

• MCST_ST8_snapshot_z3 loads with zero non-success unit states (default unit file path).
• MCST_ST8_group_z3 loads with zero non-success unit states.
• MCST_ST15_snapshot_z5p5 loads with zero non-success unit states.
• MCST_ST15_group_z5p5 loads with zero non-success unit states.
• check_unit_mismatch direct unit tests: 'none' vs Q(1, dimensionless) returns True; 'none' vs Q(2, dimensionless) and 'none' vs Q(1, km) still log mismatches.
• After the code_time fix, code_time / (UnitLength_in_cm / UnitVelocity_in_cm_per_s) = 1.000000 exactly.
• CI passes on the registered MCST_ST15 entries (relies on the test-data being mirrored to the CI testdata path).

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