feat: v4.5.0

[pocketken]

H3.net 4.5.0 'better late than never': upstream alignment, modern targets, structs, perf.

Aligns the port with upstream H3 4.5.0 and modernizes the codebase; see the 4.5.0 entry in CHANGES.md for the full breakdown.

• retarget to net10.0/net8.0/netstandard2.1/netstandard2.0; drop EOL TFMs; netstandard targets carry no System.Text.Json dependency (Unity-consumable; JSON support on net8.0+) (Add .NET 7 target #98, is .dll available to use in Unity? #140)
• fix IsValidCell 32-bit shift wraparound rejecting res 11+ cells (IsValid returns false for valid value (4.0.0) #111) and EdgeLength* silently returning 0 for non-edge input (EdgeLengthMeters returning 0 #109), plus the polar pentagon 'is not 118 or 8' precedence bug, stale 3.x average edge/area tables and other latent issues
• port upstream 4.0.1-4.5.0: CellToChildPos/ChildPosToCell/ CellToChildrenSize, pentagon-safe GridRing, bidirectional GridPathCells, Cagnoli/Kahan cell areas, IsValidIndex, ReverseDirectedEdge, CellsToMultiPolygon, grid statistics, overflow/OOB fixes and upstream test suites (Align with upstream 4.1.0 Release #102)
• convert H3Index, CoordIJK, FaceIJK and LatLng to structs (using struct instead of sealed class? #93): FromLatLng/ToLatLng measured allocation-free; remove GeoCoord and scratch-buffer parameters; annotate pure members readonly
• polyfill improvements (Polyfill Improvements #53): geometry-type dispatch incl. GeometryCollection, multi-seed disjoint MultiPolygon fills, containment predicate overload
• canonical cell sets (Add cell canonicalization (upstream RFC) #61) and component-based, validated H3Index.Create (upstream constructCell)
• allocation-free hot paths (CoordIJK->Direction ~10x, IsNeighbour fast path ~2.7x), incremental source generators, Polyfill package for netstandard backfills, updated dependencies and CI, standalone 4.0.0-vs-current benchmark suite (test/H3.CompareBenchmarks)

Closes GH-3, closes GH-44, closes GH-53, closes GH-61, closes GH-86, closes GH-93, closes GH-98 (no longer applicable -- .NET 10 now), closes GH-102 (out of date: now aligned with 4.5.0 release), closes GH-109, closes GH-111 AND closes GH-140.