Split develop.rs into develop/ by area of behaviour
develop.rs had grown to 9,327 lines covering everything the develop session does: opening a photograph, the parameter-row and curve-widget panel model, mask viewing and editing, mask creation and the rasteriser that turns a mask stack into GPU arrays, spot repairs, scene segmentation, framing and zoom, white-balance sampling, rendering and film choice, and the undo/snapshot history. docs/dev/code-health.md CH-1 names dr-ui's lack of a view layer as the reason every feature kept landing in a handful of files; this is the first of the two pure splits it recommends as easy, no-behaviour-change wins independent of that larger rework. The boundaries follow the file's own sections (several were already marked off with comment headers) and the seams a full read turned up underneath them -- mask storage/rasterisation turned out to be a distinct concern from mask viewing and editing, and rows/tabs/curves from each other, so those split further than the headers alone suggested. Each module stays under about 1,500 lines. Struct fields and the handful of helper methods now called from a sibling module became `pub(super)`, which is strictly narrower than the whole-crate reachability a single file gave them; nothing gained visibility outside `develop`. Tests moved with the code they test, including the few cases where a helper one file's tests needed was itself only defined in another's -- those became shared fixtures in `mod.rs` alongside the `headless`/`read_back`/`grey_session` helpers that already worked that way. `mod.rs` re-exports every item `develop::` callers outside this module used before, so lib.rs, masks_ui.rs and the rest needed no changes.
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