perf(docs-tests): shard route walk to drop CI wall-clock

[ottobolyos]

Summary

Implements a four-way shard of the RouteCheckTests Playwright route walk so the build-test-coverage Ubuntu leg drops from a single sequential walk of every markdown-backed route to four parallel walks of a quarter each. The longest single shard becomes the critical-path wall-clock floor rather than the whole walk.

• Add RouteCheckHelpers.ShardRoutes(routes, index, total) implementing round-robin distribution: route at position i lands in shard (i % total) + 1. Round-robin keeps per-shard wall-clock balanced even when expensive routes cluster in the source markdown tree (e.g. the generated reference pages under docs/reference/).
• Add RouteCheckHelpers.ReadShardEnv() reading ROUTE_SHARD_INDEX / ROUTE_SHARD_TOTAL from the process environment with (1, 1) as the unset / unparseable default. Local dotnet test invocations without matrix-injected env vars walk every route, identity-shard.
• Wire Every_Markdown_Backed_Route_Resolves_Without_A_404 to slice the collected route list through ShardRoutes before walking it. A surplus shard (more shards than routes) walks zero routes and short-circuits to success.
• Add shard / shardTotal matrix dimensions to .github/workflows/dotnet.yml. shard is [1, 2, 3, 4] on ubuntu-latest; windows-latest is capped at shard 1 of 4 via exclude rules because the Windows leg filters out Category=E2E and has no shardable work. Each leg env-injects ROUTE_SHARD_INDEX / ROUTE_SHARD_TOTAL from the static matrix integers (never from github.event.* untrusted input), names its TRX with the shard suffix to avoid collisions, and uploads its artifacts under a shard-qualified name.