docs: add trim-to-count design spec

Replace expensive ShrinkFiller re-fill loop with axis-aware edge-sorted
trim. Also replaces blind Take(N) in DefaultNestEngine.Fill.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-19-trim-to-count-design.md

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+# Trim-to-Count: Replace ShrinkFiller Loop with Edge-Sorted Trim
+
+## Problem
+
+When a fill produces more parts than needed, `ShrinkFiller` iteratively shrinks the work area and re-fills from scratch until the count drops below target. Each iteration runs the full fill pipeline (pairs, bestfit, linear), making this expensive. Meanwhile, `DefaultNestEngine.Fill` trims excess parts with a blind `Take(N)` that ignores spatial position.
+
+## Solution
+
+Add `ShrinkFiller.TrimToCount` — a static method that sorts parts by their trailing edge and removes from the far end until the target count is reached. Replace the shrink loop and the blind `Take(N)` with calls to this method.
+
+## Design
+
+### New method: `ShrinkFiller.TrimToCount`
+
+```csharp
+internal static List<Part> TrimToCount(List<Part> parts, int targetCount, ShrinkAxis axis)
+```
+
+- Returns input unchanged if `parts.Count <= targetCount`
+- Sorts by trailing edge descending:
+  - `ShrinkAxis.Width` → sort by `BoundingBox.Right`
+  - `ShrinkAxis.Height` → sort by `BoundingBox.Top`
+- Removes parts from the far end until `targetCount` remains
+- Returns a new list (does not mutate input)
+
+### Changes to `ShrinkFiller.Shrink`
+
+Replace the iterative shrink loop:
+
+1. Fill once using existing `EstimateStartBox` + fallback logic (unchanged)
+2. If count exceeds `shrinkTarget`, call `TrimToCount(parts, shrinkTarget, axis)`
+3. Measure dimension from trimmed result via existing `MeasureDimension`
+4. Report progress once after trim
+5. Return `ShrinkResult`
+
+The `maxIterations` parameter becomes unused and can be removed.
+
+### Changes to `DefaultNestEngine.Fill`
+
+Replace line 55-56:
+
+```csharp
+// Before:
+if (item.Quantity > 0 && best.Count > item.Quantity)
+    best = best.Take(item.Quantity).ToList();
+
+// After:
+if (item.Quantity > 0 && best.Count > item.Quantity)
+    best = ShrinkFiller.TrimToCount(best, item.Quantity, ShrinkAxis.Width);
+```
+
+Defaults to `ShrinkAxis.Width` (trim by right edge) since this is the natural "end of nest" direction outside of a shrink context.
+
+## Design Decisions
+
+- **Axis-aware trimming**: Height shrink trims by top edge, width shrink trims by right edge. This respects the strip direction.
+- **No pair integrity**: Trimming may split interlocking pairs. This is acceptable because if the layout is suboptimal, a better candidate will replace it during evaluation.
+- **No edge spacing concerns**: The new dimension is simply the max edge of remaining parts. No snapping to spacing increments.
+
+## Files Changed
+
+- `OpenNest.Engine/Fill/ShrinkFiller.cs` — add `TrimToCount`, replace shrink loop, remove `maxIterations`
+- `OpenNest.Engine/DefaultNestEngine.cs` — replace `Take(N)` with `TrimToCount`
+- `OpenNest.Tests/ShrinkFillerTests.cs` — update tests for new behavior, add `TrimToCount` tests