feat(paint): add Fill Holes segmentation process

[PaulHax]

Summary

Adds Fill Holes, a third paint "process" alongside Fill Between and Smooth. It
finds background regions fully enclosed by segmentation on a 2D slice and fills them
in — a common cleanup after painting leaves interior gaps. Fill Holes is the first
option in the process list and the default selection.

Behavior

• Per-slice flood fill on the plane of the active 2D view: anything not reachable from
  the slice border is a hole.
• Only background (0) voxels are ever filled. Existing segments are never overwritten,
  even when fully enclosed.
• Two independent options:
  • Slices — Current slice (default), or All slices (every slice along the active
    view's axis).
  • Segments — All segments (default), or Selected segment.
• All segments: each hole is filled with the majority bordering label (ties broken
  by the lowest label for determinism). Locked segments are never grown.
• Selected segment: only the active segment's holes are filled.

UX

The Preview step's Original/Processed control toggles in place — clicking either
button flips the view, so you can compare without moving the pointer. The active button
still indicates which view you're on.

Implementation

• core/tools/paint/fillHoles.ts — pure, dependency-free flood fill (unit-testable,
  worker-ready), run in a web worker (fillHoles.worker.ts) so whole-volume fills don't
  block the UI, matching Gaussian Smooth.
• store/tools/fillHoles.ts — resolves the IJK slice axis from the active 2D view and
  the segment group's parent image, and the current slice from the view's slice
  config; requires an active 2D view.
• Conforms to the existing ProcessAlgorithm contract; no changes to the process
  framework's preview/apply/cancel flow.
• Process wiring (label, icon, controls, algorithm) is centralized in a small
  processes.ts registry, so the selector and controls render from one table instead of
  per-type copy-paste.

Shared fixes (benefit all paint processes)

Review surfaced bugs in the shared process flow, fixed here so Fill Between and Smooth
benefit too:

• Apply no longer discards the result when the user is viewing the Original preview
  (previously confirmProcess committed the original scalars).
• The active-segment requirement is now opt-out, so an all-segments process isn't
  blocked by (or limited to) a single active segment.

Testing

• 11 unit tests for the core algorithm: single/all-segment fills, border handling,
  no-mutation, deterministic tie-break, locked-segment protection, whole-volume on a
  non-default axis, single-slice.
• 3 DOM-only e2e specs (default run, whole-volume + selected-segment, in-place preview
  toggle), verified against a production build in Chrome.

Manual QA

Load a dataset, create a segment group, paint a shape with an interior gap, then
Paint → Process → Fill Holes. Try each Slices/Segments combination, Preview, then
Apply or Cancel.

Out of scope

• True 3D cavity filling and multi-axis fills (whole-volume is per-slice along the
  active view's axis only).

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