perf(fill): prepare overlap polygons once per check

Both HasOverlappingParts loops rebuilt each part's polygon from its
Program on every pair. PartOverlapChecker prepares each distinct Program
(reference identity) once and each part's world polygon once per call,
then uses the overlap-only Collision.HasOverlap. Loop order, bounding-box
prefilter, early exit and returned indices are unchanged; Part.Intersects
shares the material/polygon recipe and still returns crossing points.

Verification:
- Frozen LegacyPartOverlap differential (original Intersects and both
  loops): verdicts, indices and world polygons bit-identical across fill
  grids, patterns, touching/epsilon gaps, scribe/rapid/empty programs.
- Debug OverlapPolygonPreparations: 246 -> 1 and 64 -> 2 per check.
- Corpus job (169 parts, --engines Default --parallel 1, with 1a):
  median 18,885 -> 13,464 ms over 4+4 alternating runs, identical
  outcomes; serialized layout byte-identical to the base.

AGENTS.md

@@ -141,7 +141,7 @@ Always keep `README.md` and `AGENTS.md` up to date when making changes that affe
 - Angles throughout the codebase are in **radians** (use `Angle.ToRadians()`/`Angle.ToDegrees()` for conversion).
 - `Tolerance.Epsilon` is used for floating-point comparisons across geometry operations.
 - Nesting uses async progress/cancellation: `IProgress<NestProgress>` and `CancellationToken` flow through the engine to the UI's `NestProgressForm`.
-- **Spacing offsets**: polygon consumers (`PolygonHelper`, `PartBoundary`, `NestValidator`, `CutOff`, the `LayoutPart` Draw Offset display) use `ClipperBridge.Offset`/`OffsetPerimeter`: one Clipper pass over the flattened region (perimeter positive, cutouts negative) with round joins at 1e-4 precision, so features narrower than twice the spacing collapse and closed-up holes disappear. `circumscribe: true` is the conservative mode (perimeter arcs circumscribed with endpoints kept on the arc, cutout arcs inscribed, inflation padded by the join chord error) and never under-estimates the spacing. `NestValidator` uses `OffsetForValidation` instead: the same flattening with fine joins and no padding, inflated by the spacing less `NestTolerances.SpacingSlack` (0.0005), so a layout exactly at the spacing passes even after rotation and coordinate rounding leave it ~1e-4 short. `NestJobPlacementValidator` applies the same slack to its edge-distance check. `PartGeometry.GetOffsetPerimeterEntities`/`GetOffsetPartEntities` stay on the arc-preserving per-entity `Shape.OffsetOutward`/`OffsetInward` (internal) because directional-distance loops are much faster on native arcs; their chains are closed but may keep zero-area spikes inside the envelope. `FillLinear` prepares each distinct `Program` (reference identity) once per public `Fill`/`FillRow` call and translates clones; never share that cache across calls or threads. Clipper is allowed only for cached CPU preparation, never in per-pair hot loops.
+- **Spacing offsets**: polygon consumers (`PolygonHelper`, `PartBoundary`, `NestValidator`, `CutOff`, the `LayoutPart` Draw Offset display) use `ClipperBridge.Offset`/`OffsetPerimeter`: one Clipper pass over the flattened region (perimeter positive, cutouts negative) with round joins at 1e-4 precision, so features narrower than twice the spacing collapse and closed-up holes disappear. `circumscribe: true` is the conservative mode (perimeter arcs circumscribed with endpoints kept on the arc, cutout arcs inscribed, inflation padded by the join chord error) and never under-estimates the spacing. `NestValidator` uses `OffsetForValidation` instead: the same flattening with fine joins and no padding, inflated by the spacing less `NestTolerances.SpacingSlack` (0.0005), so a layout exactly at the spacing passes even after rotation and coordinate rounding leave it ~1e-4 short. `NestJobPlacementValidator` applies the same slack to its edge-distance check. `PartGeometry.GetOffsetPerimeterEntities`/`GetOffsetPartEntities` stay on the arc-preserving per-entity `Shape.OffsetOutward`/`OffsetInward` (internal) because directional-distance loops are much faster on native arcs; their chains are closed but may keep zero-area spikes inside the envelope. `FillLinear` prepares each distinct `Program` (reference identity) once per public `Fill`/`FillRow` call and translates clones; never share that cache across calls or threads. Both `HasOverlappingParts` loops use one `PartOverlapChecker` per call (same keying; parts and programs must not change while it is in use). Clipper is allowed only for cached CPU preparation, never in per-pair hot loops.
 - **Marks are not material**: scribe/etch moves are marked on the surface, never cut through, so they are left out of nesting. `SpecialLayers.IsMaterial(layer)` (excludes `Rapid` and `Scribe`) is the filter for every consumer that builds part material from a program: drawing area, canonical angle, part collision, `PartGeometry`, plate perimeters, best-fit/pair evaluation, rotation analysis, the GPU evaluators, and both validators (`NestJobPlacementValidator`, benchmark `NestValidator`). Cutting time, on-screen display, splitting, and post-processors still see marks. Older `.nest` files (e.g. `tools/PepNestExport` output) saved etch as cut moves while their source entities kept the `SCRIBE` layer; `NestReader` runs `ScribeLayerRepair` on load to move matching program moves back to `Scribe`.
 - `Compactor` performs post-fill gravity compaction — after filling, parts are pushed toward a plate edge using directional distance calculations to close gaps between irregular shapes.
 - `FillScore` uses lexicographic comparison (count > utilization > compactness) to rank fill results consistently across all fill strategies. After its null/empty guards, `DefaultFillComparer` decides unequal counts without scoring; equal counts still use scores, and exact ties retain the current layout. `FillHelpers.FillPattern` computes eager scores only when no custom comparer is supplied; custom comparers remain authoritative and may perform their own scoring.