docs: address spec review feedback for abstract nest engine

Fix MainForm callsite descriptions, clarify default implementations
return empty lists, make FillExact non-virtual, document PackArea
signature refactor, add AutoNester scope note, specify error handling
for plugin loading, document thread safety and instance lifetime.

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

docs/superpowers/specs/2026-03-15-abstract-nest-engine-design.md

@@ -24,18 +24,22 @@ NestEngineRegistry (static, OpenNest.Engine)
 └── Factory method: Create(Plate) → NestEngineBase
 ```
 
+**Note on AutoNester:** The existing `AutoNester` static class (NFP + simulated annealing for mixed parts) is a natural future candidate for the registry but is currently unused by any caller. It is out of scope for this refactor — it can be wrapped as an engine later when it's ready for use.
+
 ## NestEngineBase
 
 Abstract base class in `OpenNest.Engine`. Provides the contract, shared state, and utility methods.
 
+**Instance lifetime:** Engine instances are short-lived and plate-specific — created per operation via the registry factory. Some engines (like `DefaultNestEngine`) maintain internal state across multiple `Fill` calls on the same instance (e.g., `knownGoodAngles` for angle pruning). Plugin authors should be aware that a single engine instance may receive multiple `Fill` calls within one nesting session.
+
 ### Properties
 
 | Property | Type | Notes |
 |----------|------|-------|
 | `Plate` | `Plate` | The plate being nested |
 | `PlateNumber` | `int` | For progress reporting |
-| `NestDirection` | `NestDirection` | Fill direction preference |
-| `WinnerPhase` | `NestPhase` | Which phase produced the best result (private set) |
+| `NestDirection` | `NestDirection` | Fill direction preference, set by callers after creation |
+| `WinnerPhase` | `NestPhase` | Which phase produced the best result (protected set) |
 | `PhaseResults` | `List<PhaseResult>` | Per-phase results for diagnostics |
 | `AngleResults` | `List<AngleResult>` | Per-angle results for diagnostics |
 
@@ -48,7 +52,7 @@ Abstract base class in `OpenNest.Engine`. Provides the contract, shared state, a
 
 ### Virtual Methods (return parts, no side effects)
 
-These are the core methods subclasses override:
+These are the core methods subclasses override. Base class default implementations return empty lists — subclasses override the ones they support.
 
 ```csharp
 virtual List<Part> Fill(NestItem item, Box workArea,
@@ -57,14 +61,13 @@ virtual List<Part> Fill(NestItem item, Box workArea,
 virtual List<Part> Fill(List<Part> groupParts, Box workArea,
     IProgress<NestProgress> progress, CancellationToken token)
 
-virtual List<Part> FillExact(NestItem item, Box workArea,
-    IProgress<NestProgress> progress, CancellationToken token)
-
 virtual List<Part> PackArea(Box box, List<NestItem> items,
     IProgress<NestProgress> progress, CancellationToken token)
 ```
 
-Default implementations in the base class delegate to `DefaultNestEngine`'s logic (or return empty lists — see DefaultNestEngine below).
+**`FillExact` is non-virtual.** It is orchestration logic (binary search wrapper around `Fill`) that works regardless of the underlying fill algorithm. It lives in the base class and calls the virtual `Fill` method. Any engine that implements `Fill` gets `FillExact` for free.
+
+**`PackArea` signature change:** The current `PackArea(Box, List<NestItem>)` mutates the plate directly and returns `bool`. The new virtual method adds `IProgress<NestProgress>` and `CancellationToken` parameters and returns `List<Part>` (side-effect-free). This is a deliberate refactor — the old mutating behavior moves to the convenience overload `Pack(List<NestItem>)`.
 
 ### Convenience Overloads (non-virtual, add parts to plate)
 
@@ -95,7 +98,9 @@ Rename of the current `NestEngine`. Inherits `NestEngineBase` and overrides all
 
 - `Name` → `"Default"`
 - `Description` → `"Multi-phase nesting (Linear, Pairs, RectBestFit, Remainder)"`
-- All current private methods (`FindBestFill`, `FillWithPairs`, `FillRectangleBestFit`, `FillPattern`, `TryRemainderImprovement`, `BuildCandidateAngles`, etc.) remain as private methods in this class
+- All current private methods (`FindBestFill`, `FillWithPairs`, `FillRectangleBestFit`, `FillPattern`, `TryRemainderImprovement`, `BuildCandidateAngles`, `QuickFillCount`, etc.) remain as private methods in this class
+- `ForceFullAngleSweep` property stays on `DefaultNestEngine` (not the base class) — only used by `BruteForceRunner` which references `DefaultNestEngine` directly
+- `knownGoodAngles` HashSet stays as a private field — accumulates across multiple `Fill` calls for angle pruning
 - No behavioral change — purely structural refactor
 
 ## StripNestEngine
@@ -109,7 +114,7 @@ The planned `StripNester` (from the strip nester spec) becomes a `NestEngineBase
 
 ## NestEngineRegistry
 
-Static class in `OpenNest.Engine` managing engine discovery and selection.
+Static class in `OpenNest.Engine` managing engine discovery and selection. Accessed only from the UI thread — not thread-safe. Engines are created per-operation and used on background threads, but the registry itself is only mutated/queried from the UI thread at startup and when the user changes the active engine.
 
 ### NestEngineInfo
 
@@ -146,7 +151,13 @@ Follows the existing `IPostProcessor` pattern from `Posts/`:
 - Reflect over types, find concrete subclasses of `NestEngineBase`
 - Require a constructor taking `Plate`
 - Register each with its `Name` and `Description` properties
-- Called at application startup alongside post-processor loading
+- Called at application startup alongside post-processor loading (WinForms app only — Console and MCP use built-in engines only)
+
+**Error handling:**
+- DLLs that fail to load (bad assembly, missing dependencies) are logged and skipped
+- Types without a `Plate` constructor are skipped
+- Duplicate engine names: first registration wins, duplicates are logged and skipped
+- Exceptions from plugin constructors during `Create()` are caught and surfaced to the caller
 
 ## Callsite Migration
 
@@ -154,7 +165,7 @@ All `new NestEngine(plate)` calls become `NestEngineRegistry.Create(plate)`:
 
 | Location | Count | Notes |
 |----------|-------|-------|
-| `MainForm.cs` | 3 | Auto-nest, fill plate, fill area |
+| `MainForm.cs` | 3 | Auto-nest fill, auto-nest pack, single-drawing fill plate |
 | `ActionFillArea.cs` | 2 | |
 | `PlateView.cs` | 1 | |
 | `NestingTools.cs` (MCP) | 6 | |