feat(engine): add built-in Rectangles and Irregular nesting engines

Moves the two production plug-in engines into OpenNest.Engine under names
that describe the jobs they suit:

- Rectangles: plain and near-rectangular plates, maximal-rectangles box
  packing (was the RectanglesNestingEngine plug-in)
- Irregular: irregular profiles, no-fit-polygon frontier packing (was the
  Opus55NestingEngine plug-in)

Their tests and the shared engine contract/layout test kit move into
OpenNest.Engine.Tests/NestingEngines.

The registry maps the old plug-in names to the new engines, so saved desktop
selections, scripts and API requests keep working, and a leftover plug-in DLL
under an old name cannot shadow its replacement. Desktop startup passes the
registry's lookup when restoring the saved Auto Nest engine.

OpenNest.Engine/NestingEngines/Rectangles/RectanglesNestingEngine.cs

@@ -0,0 +1,154 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using OpenNest.Engine.Jobs;
+
+namespace OpenNest.Engine.NestingEngines.Rectangles;
+
+/// <summary>
+/// Rectangle-lane nesting engine: every part is nested as the axis-aligned box of its material at
+/// its minimum-area rotations, packed with a maximal-rectangles free list (Jylänki 2010).
+///
+/// Built for jobs of plain and near-rectangular parts, where a part's box wastes almost nothing
+/// and exact box packing beats contour-sliding engines on both speed and density. Irregular parts
+/// are still placed validly, only as their bounding boxes; they are not nested into each other.
+///
+/// Sheet by sheet, each available stock is packed under several free-space scoring rules and
+/// two pick modes (best-fitting box anywhere, or largest type first). The candidate sheet with
+/// the lowest estimated whole-job cost wins: its salvage-credited net area (NestJobCost) plus the
+/// remaining demand priced at the best net-area-per-part-area ratio seen among the candidates.
+/// Deterministic: no clocks or randomness; the only stop besides completion is the host token.
+/// </summary>
+public sealed class RectanglesNestingEngine : INestingEngine
+{
+    private static readonly FitRule[] Rules =
+    {
+        FitRule.BestShortSide, FitRule.BestLongSide, FitRule.BestArea,
+        FitRule.BottomLeft, FitRule.LeftBottom, FitRule.ContactPoint,
+    };
+
+    private static readonly PickMode[] Modes = { PickMode.Global, PickMode.Ordered };
+
+    public NestJobResult Solve(
+        NestJob job,
+        IProgress<NestJobProgress>? progress = null,
+        CancellationToken token = default
+    )
+    {
+        ArgumentNullException.ThrowIfNull(job);
+        token.ThrowIfCancellationRequested();
+
+        var types = BoxCatalog.Build(job);
+        var remaining = types.Select(t => t.Part.Quantity).ToArray();
+        // Parts with unreadable geometry or no box that fits any offered sheet can never be placed.
+        foreach (var t in types)
+            if (t.Orientations.Count == 0 || !job.Plates.Any(stock => t.Orientations.Any(o => FitsStock(stock, o))))
+                remaining[t.Index] = 0;
+
+        var used = job.Plates.ToDictionary(s => s.Id, _ => 0, StringComparer.Ordinal);
+        var result = new NestJobResultBuilder(job, progress);
+        NestJobStopReason reason;
+
+        while (true)
+        {
+            if (remaining.All(r => r == 0))
+            {
+                reason = NestJobStopReason.NoPlacementFound; // Builder reports Completed when demand is met.
+                break;
+            }
+            if (job.Options.MaxPlates is int cap && result.SheetsUsed(job) >= cap)
+            {
+                reason = NestJobStopReason.PlateLimitReached;
+                break;
+            }
+
+            var trials = new List<(SheetPlan Plan, double Net)>();
+            foreach (var stock in job.Plates)
+            {
+                token.ThrowIfCancellationRequested();
+                if (stock.Quantity is int available && used[stock.Id] >= available)
+                    continue;
+                progress?.Report(new NestJobProgress(
+                    NestJobStage.EvaluatingCandidate, stock.Id, result.SheetsUsed(job), result.SheetsUsed(job), 0));
+                foreach (var mode in Modes)
+                    foreach (var rule in Rules)
+                    {
+                        var plan = SheetPacker.Pack(types, remaining, stock, rule, mode, token);
+                        if (plan.Parts.Count > 0)
+                            trials.Add((plan, NetArea(job, plan)));
+                    }
+            }
+
+            if (trials.Count == 0)
+            {
+                var exhausted = job.Plates.Any(s => s.Quantity is int q && used[s.Id] >= q);
+                reason = exhausted ? NestJobStopReason.StockExhausted : NestJobStopReason.NoPlacementFound;
+                break;
+            }
+
+            var chosen = Choose(types, remaining, trials);
+            result.AddSheet(chosen.Stock, chosen.Poses());
+            used[chosen.Stock.Id]++;
+            foreach (var p in chosen.Parts)
+                remaining[p.Type.Index]--;
+        }
+
+        return result.Build(reason);
+    }
+
+    /// <summary>
+    /// Picks the sheet with the lowest estimated whole-job cost. Remaining demand is priced at the
+    /// best net-area-per-material ratio any candidate achieved, so a sheet that finishes the job
+    /// competes fairly with a denser partial one. Ties: more material placed, then enumeration order.
+    /// </summary>
+    private static SheetPlan Choose(
+        IReadOnlyList<BoxType> types, int[] remaining, List<(SheetPlan Plan, double Net)> trials)
+    {
+        var demandArea = types.Sum(t => remaining[t.Index] * t.MaterialArea);
+        var bestRatio = trials.Min(t => t.Net / System.Math.Max(t.Plan.MaterialArea, 1e-12));
+        return trials
+            .Select((t, order) => (t.Plan, order,
+                Estimate: t.Net + System.Math.Max(0, demandArea - t.Plan.MaterialArea) * bestRatio))
+            .OrderBy(t => PriorityDebt(types, remaining, t.Plan))
+            .ThenBy(t => t.Estimate)
+            .ThenByDescending(t => t.Plan.MaterialArea)
+            .ThenBy(t => t.order)
+            .First()
+            .Plan;
+    }
+
+    /// <summary>
+    /// Priority guard: how many instances of the most urgent (lowest-number) tier with remaining
+    /// demand this plan leaves unplaced. Plans are ranked on this before cost, so a cheaper sheet
+    /// can never win by serving a later tier at the expense of an earlier one.
+    /// </summary>
+    private static int PriorityDebt(IReadOnlyList<BoxType> types, int[] remaining, SheetPlan plan)
+    {
+        var active = types.Where(t => remaining[t.Index] > 0).ToList();
+        if (active.Count == 0)
+            return 0;
+        var top = active.Min(t => t.Priority);
+        var placed = plan.Parts.Count(p => p.Type.Priority == top);
+        return active.Where(t => t.Priority == top).Sum(t => remaining[t.Index]) - placed;
+    }
+
+    private static double NetArea(NestJob job, SheetPlan plan) =>
+        plan.Envelope is { } envelope
+            ? NestJobCost.NetSheetArea(job.Options, plan.Stock, envelope)
+            : plan.Stock.Area;
+
+    private static bool FitsStock(NestPlateStock stock, BoxOrientation o)
+    {
+        var work = stock.WorkArea;
+        return o.Width <= work.Right - work.Left + MaxRectsSheet.Eps
+            && o.Height <= work.Top - work.Bottom + MaxRectsSheet.Eps;
+    }
+}
+
+internal static class ResultBuilderExtensions
+{
+    public static int SheetsUsed(this NestJobResultBuilder builder, NestJob job) =>
+        job.Plates.Sum(builder.SheetsUsed);
+}