Split pipeline into cacheable segment + re-runnable finish

Pipeline::segment runs only the frontend (the expensive clustering) and
returns a reusable Segmentation; Pipeline::finish re-runs just color
fitting, compositing, and optimization over a cached segmentation. This
restores the old stage-reuse workflow: cluster once, then tune curve-fit
or color-fit params without repaying clustering. Both have
*_with_progress variants; run/run_with_progress now compose the two
(one-shot path moves the owned segmentation, so it adds no clone).

finish clones the segmentation internally (color fitting mutates it), so
the cached copy stays pristine across many finish calls. Segmentation and
VectorDoc are re-exported at the crate root.

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 * Progress reporting and cancellation: `Pipeline::run_with_progress` with a `CancelToken` and a per-phase progress callback (for driving desktop UIs from a worker thread).
+* Two-phase conversion for interactive tuning: `Pipeline::segment` caches the expensive clustering result as a reusable `Segmentation`, and `Pipeline::finish` re-runs only the cheap color-fitting / curve-fitting / optimization stages — so tuning those parameters no longer repays the clustering cost. Both have `*_with_progress` variants.
 * Binary thresholding methods: a tunable fixed threshold and Bradley–Roth adaptive thresholding (via visioncortex's summed-area table) for images with uneven lighting. Exposed on `Config` (`binary_threshold`, `binary_adaptive`, `binary_adaptive_window`, `binary_adaptive_t`), the CLI (`--threshold`, `--adaptive`, `--adaptive-window`, `--adaptive-t`), Python, and the Node package (`binaryThreshold`, `adaptive`, `adaptiveWindow`, `adaptiveT`).
 
 ## 1.0.0-alpha.1 - 2026-07-24