Preserve authored nested dataclass types in clean generated source

[trissim]

Clean mode silently dropped an authored nested dataclass subtype when all of the subtype's own fields matched its defaults. The parent's default constructor then recreated a different nominal type. An OpenHCS configuration containing VolumeSourceSpatialDomain() consequently rendered as GlobalPipelineConfig() and reopened as an XY declaration.

The existing generic formatter only omits an empty nested constructor when its nominal owner matches the field's default owner. Ordinary unchanged defaults still disappear from clean source. No application-specific formatter or subtype roster is added.

Validation: the complete existing suite plus three focused controls passed (12 tests). Two controls reproduced the loss before the fix, covering both extra subtype fields and a semantic subtype with no additional fields. Actual public OpenHCS ConfigDocument roundtrips retain both default-valued and explicit-depth volume declarations. An actual original 31-step 3D PipelineDocument source roundtrip also preserves the intrinsic type and all three image source bindings, jointly checked through ObjectState's independent runtime-owner fix.

Joint production qualification: the full original 31-step 3D pipeline passes public source decoding, orchestrator initialization using its actual 180-reference source index, and compilation to CompiledExecutionBundle on OpenHCS 2b9bb5649. No pipeline execution or performance result is claimed.

Closes #1

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