perf(fill): reuse offset geometry for translated copies

FillLinear re-prepared offset perimeter geometry (ConvertProgram ->
ShapeProfile -> OffsetOutward) for every part it measured, although
tiled copies share one Program and differ only by Location. A CPU
profile of a 169-part Default job put 62% of wall time there.

Prepare each distinct Program (reference identity) once per public
Fill/FillRow call in local frame, then clone and translate for each
location. The cache is created per call and passed down privately
because FillHelpers.FillPattern calls Fill concurrently on one
instance. PartGeometry gains a local-frame Program overload that the
Part overload now delegates to.

Evaluation order, lazy preparation, fallbacks and tiling are
unchanged. Differential tests against a frozen copy of the previous
FillLinear check bitwise equality, including concurrent calls; Debug
work tests pin preparation counts. With the thread pool capped at one
worker, before/after whole-job layouts are byte-identical. The
Default corpus job median drops from 40,715 to 18,810 ms.

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. 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. 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.