refactor(benchmarks): Spectre.Console CLI + project restructure

[migus88]

Summary

Tidies up the Migs.MPath.Benchmarks project and modernizes how it's run.

• Spectre.Console.Cli front end — Program.cs is now a thin CommandApp; the old if/else argument chain is gone. Commands live in Commands/ (benchmark, render, info), with auto-generated --help and friendly validation errors.
• Reorganized layout:
  • Suites/ — the BenchmarkDotNet [Benchmark] classes
  • Competitors/ — rival A* library adapters (IMazeBenchmarkRunner); was MazeBenchmarks/
  • Common/ — shared BenchmarkAgent + BenchmarkScenario (maze path + canonical start/destination)
• CWD-independent — BenchmarkScenario resolves the maze and Results/ paths against the assembly base directory, so every command works from any working directory (previously debug/render only worked from inside the project folder).
• Re-enabled competitor maze benchmarks (RoyT, AStarLite, LinqToAStar).
• Added a reachability suite (ReachabilityBenchmarkRunner) benchmarking Pathfinder.GetReachable across small/medium/large budgets.
• Rider run configs fixed — removed the stale Atomic.Pathfinding (pre-rename) build method and obsolete configs, added a Benchmark Reachability config, and corrected a stale .idea .iml ignore entry.
• Misc: fixed the RoyTAStarMazeB*u*nchmarkRunner.cs filename typo, dropped the stale cavern.gif csproj reference, suppressed a benign transitive-package TFM warning.

⚠️ Merge order / dependency

ReachabilityBenchmarkRunner calls Pathfinder.GetReachable, which is not on master yet (it ships in the separate GetReachable PR). This branch will not build until GetReachable lands, so CI here is expected to be red until then. Everything else in the restructure compiles cleanly — the only build errors are the three GetReachable references.

Test status

• Full solution builds except the documented GetReachable references.
• The benchmark suites, CLI commands, and renderers were exercised locally on top of the GetReachable feature and run correctly.

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