Content-aware Remove: inpaint crate (PatchMatch completion) and a Fill spot mode

New `inpaint` crate (L1): multi-scale patch-based completion with a PatchMatch
nearest-neighbour field, seeded by a membrane fill. Deterministic, no panics on
hostile sizes, cancellable. Ported from PhotoCraft (MIT OR Apache-2.0).

Remove tool: a fourth spot mode, `fill` ("Content-Aware"), rebuilds the spot
from patches of its surroundings instead of one source; coarse-to-fine, so the
structure is chosen at a scale set by the spot size and preview and export agree
(tested). spot.add leaves its source unset.

Sources: C. Barnes et al., PatchMatch (SIGGRAPH 2009); Y. Wexler, E. Shechtman,
M. Irani, Space-Time Completion of Video (TPAMI 2007); P. Perez et al., Poisson
Image Editing (SIGGRAPH 2003) for the membrane fill; code ported from PhotoCraft
crates/algo/src/inpaint.rs + poisson.rs (a680ed22).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/develop/src/settings.rs

@@ -963,6 +963,8 @@ pub enum SpotMode {
     Remove,
     Heal,
     Clone,
+    /// Content-aware: the spot is rebuilt from patches of its surroundings (no single source).
+    Fill,
 }
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]