Fix optional colony imports and 3-D rendering

[georgeoshardo]

What changed

• defer Ray and CellModeller imports until their dependent methods are called, with feature-specific errors
• write 2-D float renders as scaled 16-bit PNGs and use unique output indices
• load simple fluorescence scenes from the fluorescence directory
• compose instance masks by assignment so overlaps do not create synthetic labels and one-pixel cells remain labeled
• use independent x/y offsets for rectangular scenes
• return and crop the actual 3-D colony volume, including mixed-depth cells
• default and validate 3-D PSF working distance, and reject 3-D kernels at the 2-D convolution boundary
• preserve the z axis during colony rendering and normalize the completed volume globally

Why

The retained legacy colony paths failed to import unless Ray and CellModeller were installed, and several rendering branches could overwrite samples, lose labels or crop boundaries, return blank data, or mishandle 3-D shapes and normalization. Ray and CellModeller remain optional; this PR does not add heavyweight integration tests or dependencies.

Validation

• pixi run pytest -q tests/test_colony_optional_dependencies.py tests/test_colony_rendering.py tests/test_psf_3d.py tests/test_renderer_mask_export.py tests/test_colony_overlap.py — 19 passed
• pixi run pytest -q — 79 passed, with the existing GPU-backend fallback warning