Add GetReachable movement-range query to Pathfinder

[migus88]

What

Adds Pathfinder.GetReachable(IAgent agent, Coordinate from, float budget) — a movement-range query that returns every cell whose cheapest path cost from the origin is within a budget. This is the core mechanic behind tactics/strategy games that highlight where a unit can move.

It is a bounded uniform-cost (Dijkstra) flood fill that reuses the existing infrastructure:

• Same movement rules as GetPath — diagonal movement, corner-cutting, occupied cells, and agent clearance (Size > 1).
• Same pooled, reusable Cell[] buffer and unsafe hot path.

Per-step cost is StraightMovementMultiplier (cardinal) / DiagonalMovementMultiplier (diagonal), multiplied by the destination cell's Weight when IsCellWeightEnabled is set. The origin is always included with cost 0.

Results come back as a poolable, IDisposable RangeResult of ReachableCell (Coordinate + Cost), mirroring PathResult. Contains / TryGetCost convenience lookups are provided.

using var range = pathfinder.GetReachable(agent, new Coordinate(4, 4), 5f);
foreach (var cell in range.Cells)
    Highlight(cell.Coordinate, cell.Cost);

Why

GetPath answers "how do I get from A to B?". Range/reachability ("where can I go?") is the other half of grid movement and is commonly requested for turn-based and tactics games. It can't be expressed with GetPath (no single destination), so it needs its own search.

Supporting changes

• UnsafePriorityQueue.UpdatePriority — a real decrease-key (re-heapify via cascade up/down) used by the bounded Dijkstra search. The A* hot path is unchanged.
• Fix Utils.CellsComparison to sort X-major then Y-minor so the Cell[] (CellsArray) constructor's sorted layout matches GetCell's x * Height + y indexing — consistent with the matrix/holder init modes which write cells at Coordinate.X * Height + Coordinate.Y. The previous (Y, X) ordering transposed the grid and scrambled neighbor adjacency. This was a latent bug masked in existing GetPath tests because they use symmetric (x == y) start/destination coordinates; it surfaced immediately for the range query. A dedicated asymmetric-origin test now guards against regressing it.

Verification

• dotnet build src/mpath-source/Migs.MPath.sln — succeeds, 0 errors.
• dotnet test src/mpath-source/Migs.MPath.Tests/Migs.MPath.Tests.csproj — 78/78 passing, including 13 new reachability tests (zero budget, negative budget, Manhattan diamond, diagonals, walls, cell weights, larger-agent clearance, asymmetric-origin orientation, invalid origin, null agent, disposal, instance reuse) and all pre-existing tests (confirming the CellsComparison fix introduced no regressions).

Docs

• New docs/api/RangeResult.md and docs/api/ReachableCell.md; updated docs/api/Pathfinder.md and the API index.
• New docs/guides/movement-range.md (linked from the docs index).
• Updated README.md (feature list + quick-start snippet) and CLAUDE.md (architecture notes for the new partial, data types, queue decrease-key, and the CellsComparison ordering invariant).

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