Add Manhattan/Chebyshev distance and line-of-sight public API

[migus88]

What

Adds three lightweight spatial queries to Pathfinder's public API for range and visibility logic that doesn't require computing a full path:

API	Kind	Description
static int GetManhattanDistance(Coordinate from, Coordinate to)	pure	Taxicab distance |dx| + |dy| (the A* heuristic; cardinal-only grids).
static int GetChebyshevDistance(Coordinate from, Coordinate to)	pure	Chessboard distance max(|dx|, |dy|) (8-directional grids).
bool HasLineOfSight(Coordinate from, Coordinate to, LineOfSightMode mode = BlockedByUnwalkableCells)	instance	Whether an unobstructed straight line connects two cells.

Why

These are common needs in tactics/strategy and action games — range checks, AI target sorting, fog-of-war, ranged attacks, or short-circuiting pathfinding when a target is in plain sight — that previously required either a full GetPath call or rolling your own metric.

Line-of-sight modes

HasLineOfSight takes an optional LineOfSightMode so callers can decide whether obstacles that block movement also block vision (MPath models terrain with a single IsWalkable flag):

Mode	Unwalkable cells	Occupied cells (when IsCalculatingOccupiedCells)
BlockedByUnwalkableCells (default)	block sight	block sight
IgnoreUnwalkableCells	transparent (water, pits, glass walls)	block sight

The mode governs walkability only — occupancy stays orthogonal, so "ignore terrain, but units still block the shot" is expressible.

Implementation notes

• New Pathfinder_Geometry.cs partial + LineOfSightMode enum under the shared src/mpath-unity-project/Packages/MPath/Source/ folder, so both the NuGet (Migs.MPath) and Unity (com.migsweb.mpath) packages pick them up. Kept engine-agnostic (Source/ has no UnityEngine references).
• Distance metrics are static, pure, allocation-free integer functions — they ignore walls, weights and settings and return the geometric grid distance, not a traversal cost.
• HasLineOfSight reuses the existing Bresenham line-tracing already used by string-pulling smoothing, now sharing a single mode-aware IsLineOfSightBlocked per-cell test (smoothing always passes BlockedByUnwalkableCells, so paths still can't be shortcut through walls). Endpoints are never tested, the ray is single-cell (agent size ignored), a cell always sees itself, and an out-of-range coordinate throws ArgumentException.
• No new settings, so nothing to thread through IPathfinderSettings/FastPathfinderSettings.

Verification

• dotnet build src/mpath-source/Migs.MPath.sln — succeeds (0 errors; only pre-existing warnings).
• dotnet test src/mpath-source/Migs.MPath.Tests/Migs.MPath.Tests.csproj — 100 passed, 0 failed, including 21 cases in PathfinderGeometryTests (distance correctness/symmetry/zero; LOS clear/diagonal/self, wall on vs. beside the line, endpoint exclusion, occupancy-setting behavior, the two modes incl. "ignore terrain but units still block", symmetry, repeated calls, invalid-coordinate validation).

Docs

• docs/api/Pathfinder.md — methods table + a "Distance and line of sight" section.
• docs/api/LineOfSightMode.md — new enum page (added to the API index).
• docs/guides/distance-and-line-of-sight.md — new guide (added to docs/README.md index).
• README.md — feature bullet + quick-start snippet.
• CLAUDE.md — architecture note for the new partial and enum.

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