Avoid unnecessary copies in aligned single-component mosaics

[timbeccue]

Description

MosaicCreator rebuilds the science, mask, and uncertainty arrays even when a single component already matches the output geometry. For large QHY frames, this adds unnecessary memory copying and temporary-file allocation.

This PR adds a shortcut for single-component images that already match the output geometry. It reuses existing arrays while keeping the same header updates, uncertainty precision, and memory-map behavior in later stages.

The shortcut is used for inputs with native float64 science data, uint8 masks, and writable, contiguous arrays. All other inputs use the existing mosaic code.

Validation

• 438 unit tests passed, with five end-to-end tests excluded.
• Regression coverage includes array and header equivalence, allocation counts, separate primary headers, binned windows, cropping, flipping, dtype handling, and read-only or strided inputs.
• A single local synthetic full-frame benchmark with float64 uncertainties decreased from 2.85 s to 0.076 s, with matching output arrays and headers.