docs: address spec review feedback for nesting progress window

Clarify engine return type ownership, cancellation propagation into
parallel loops, quantity decrement sequencing, conditional phase
behavior, plate navigation lockout, and MDI child close handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-03-13-nesting-progress-window-design.md

@@ -18,12 +18,12 @@ A class carrying progress updates from the engine to the UI:
 - `PlateNumber` (int): Current plate number (for auto-nest multi-plate loop)
 - `BestPartCount` (int): Part count of current best result
 - `BestDensity` (double): Density percentage of current best
-- `BestRemnantArea` (double): Usable remnant area of current best
+- `UsableRemnantArea` (double): Usable remnant area of current best (matches `FillScore.UsableRemnantArea`)
 - `BestParts` (List\<Part\>): Cloned snapshot of the best parts for preview
 
 `Phase` uses a `NestPhase` enum (defined in the same file) to prevent typos and allow the progress form to map to display-friendly text (e.g., `NestPhase.Pairs` → "Trying pairs...").
 
-`BestParts` must be a cloned list (using `Part.Clone()`) so the UI thread can safely read it while the engine continues on the background thread. Progress is reported only after each phase completes (3-4 reports per fill call), so the cloning cost is negligible.
+`BestParts` must be a cloned list (using `Part.Clone()`) so the UI thread can safely read it while the engine continues on the background thread. The clones share `BaseDrawing` references (not deep copies of drawings) since drawings are read-only templates during nesting. Progress is reported only after each phase completes (3-4 reports per fill call), so the cloning cost is negligible.
 
 ## Engine Changes
 
@@ -45,6 +45,8 @@ New signatures:
 - `List<Part> Fill(NestItem item, Box workArea, IProgress<NestProgress> progress, CancellationToken token)`
 - `List<Part> Fill(List<Part> groupParts, Box workArea, IProgress<NestProgress> progress, CancellationToken token)`
 
+**Note on `Fill(List<Part>, ...)` overload:** When `groupParts.Count > 1`, only the Linear phase runs (no RectBestFit, Pairs, or Remainder). The additional phases only apply when `groupParts.Count == 1`, matching the existing engine behavior.
+
 Inside `FindBestFill`, after each strategy completes:
 
 1. **Linear phase**: Try all rotation angles (already uses `Parallel.ForEach`). If new best found, report progress with `Phase=Linear`. Check cancellation token.
@@ -56,6 +58,8 @@ Inside `FindBestFill`, after each strategy completes:
 
 On cancellation, the engine returns its current best result (not null/empty). `OperationCanceledException` is caught internally so the caller always gets a usable result. This enables "stop early, keep best result."
 
+The `CancellationToken` is also passed into `ParallelOptions` for the existing `Parallel.ForEach` (linear phase) and `Parallel.For` (pairs phase) loops, so cancellation is responsive even mid-phase rather than only at phase boundaries.
+
 ## PlateView Temporary Parts
 
 **Modified file: `OpenNest/Controls/PlateView.cs`**
@@ -68,7 +72,7 @@ private List<LayoutPart> temporaryParts = new List<LayoutPart>();
 
 ### Drawing
 
-In `DrawParts`, after drawing real parts, iterate `temporaryParts` and draw them using a distinct preview color (semi-transparent orange or cyan). Same drawing logic, different pen/brush.
+In `DrawParts`, after drawing real parts, iterate `temporaryParts` and draw them using a distinct preview color. The preview color is added to `ColorScheme` (e.g., `PreviewPart`) so it follows the existing theming pattern. Same drawing logic, different pen/brush.
 
 ### Public API
 
@@ -124,7 +128,7 @@ The Plate row shows just the current plate number (no total — the total is not
 4. `Task.Run` with the multi-plate loop. The background work computes results only — all UI/plate mutation happens on the UI thread via `Progress<T>` callbacks and the `await` continuation:
    - The loop iterates items, calling the new `Fill(item, workArea, progress, token)` overloads which return `List<Part>` without modifying the plate.
    - Progress callbacks update the preview via `SetTemporaryParts()` on the UI thread.
-   - When a plate's fill completes, the continuation (back on the UI thread) calls `AcceptTemporaryParts()` to commit parts to the plate, decrements `NestItem.Quantity`, and calls `Nest.CreatePlate()` for the next plate if needed.
+   - When a plate's fill completes, the continuation (back on the UI thread) counts the returned parts per drawing (from the last `NestProgress.BestParts`) to decrement `NestItem.Quantity`, then calls `AcceptTemporaryParts()` to commit to the plate. `Nest.CreatePlate()` for the next plate also happens on the UI thread.
    - On cancellation, breaks out of the loop and commits whatever was last previewed.
 5. On completion, call `Nest.UpdateDrawingQuantities()`, close progress form
 6. Disable nesting-related menu items while running, re-enable on completion
@@ -136,7 +140,9 @@ Same pattern but simpler — no plate loop, single fill call with progress/cance
 
 ### UI Lockout
 
-While the engine runs, the user can pan/zoom the PlateView (read-only interaction) but editing actions (add/remove parts, change plates, etc.) are disabled. Re-enabled when nesting completes or is stopped.
+While the engine runs, the user can pan/zoom the PlateView (read-only interaction) but editing actions (add/remove parts, change plates, plate navigation) are disabled. Plate navigation is locked during auto-nest to prevent the PlateView from switching away from the plate being filled. Re-enabled when nesting completes or is stopped.
+
+If the user closes the `EditNestForm` (MDI child) while nesting is running, the cancellation token is triggered and the progress form is closed. No partial results are committed.
 
 ### Error Handling