Add stepwise A* search for visualizing pathfinding progress

[migus88]

What

Adds a stepwise (tick-by-tick) variant of the A* search for displaying how the pathfinder works — an educational/visualization facility that does not replace or change GetPath.

Pathfinder.BeginStepwiseSearch(agent, from, to) returns a disposable Pathfinder.StepwiseSearch. Each Tick() performs a single cell expansion and returns an immutable SearchStep snapshot containing:

• the accumulated searched area — every discovered cell tagged Open (frontier) or Closed (expanded), each with its A* g/h/f scores,
• Current (the cell expanded this tick), OpenCount/ClosedCount, and
• the path once the destination is reached.

RunToCompletion() advances to the end in one call.

using var search = pathfinder.BeginStepwiseSearch(agent, from, to);
while (!search.IsComplete)
{
    var step = search.Tick();
    Render(step.Searched); // open vs closed cells, with scores
}

Why this design

The request asked for a Tick-style API that shows the flow without changing how the pathfinder works today and without duplicating code — and floated "a separate class that inherits from Pathfinder".

• No subclassing. Pathfinder is sealed and its algorithm internals are private; inheriting would force unsealing it and exposing unsafe pointer internals as protected. Instead the driver is a public nested Pathfinder.StepwiseSearch, which reuses every private helper directly.
• No duplicated logic. A single A* iteration is factored into a shared private ExpandNext, called by both the batch GetPath loop and the stepwise search — so they run identical expansion logic and find the same path. ExpandNext is AggressiveInlining, keeping GetPath's hot path unchanged (a pure, behavior-preserving refactor; the existing 100 tests still pass).
• Correctness over speed, as requested for this path: the session pins the cell buffer (GCHandle) for the search lifetime so the open set's raw pointers stay valid across ticks — hence it is IDisposable and locks the instance to one active session (InvalidOperationException on a second). Snapshots own plain arrays (no pooling) so they survive across ticks.

The visualized Path is the raw A* path (no smoothing), following PathResult's origin-excluded convention.

New public types

Pathfinder.StepwiseSearch, SearchStep, SearchNode, SearchState, SearchNodeState.

Verification

• dotnet build src/mpath-source/Migs.MPath.sln — 0 errors.
• dotnet test …/Migs.MPath.Tests.csproj — 115 passed (100 existing + 15 new stepwise tests covering path equivalence with GetPath, monotonic accumulation, open/closed partitioning, failure on unreachable targets, origin==destination, idempotent/disposed ticks, the single-session guard, and argument validation).

Docs

• New docs/api/ pages: StepwiseSearch, SearchStep, SearchNode, SearchState (+ index).
• New guide docs/guides/visualizing-search.md (+ docs/README.md index).
• docs/api/Pathfinder.md, README.md, and CLAUDE.md updated for the new public surface and architecture.

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