fix: provisioner falls back to cache on timeouts, self-heals its cache dir, orders versions by SemVer 2

[Arthurvdv]

Summary

• Internal NuGet timeouts (resolve: 10s, download: 60s) now fall back to the newest valid cached analyzer version instead of leaving Ready = null for the entire session.
• A half-extracted cache directory (from a crash during a previous provisioning) is now detected and replaced on the next startup, instead of being returned as-is and causing repeated download failures.
• All three version-comparison call sites (NuGet version resolution, cache fallback, DevTools tool-store probe) now use a shared SemanticVersion type with full SemVer 2 prerelease ordering, fixing 1.3.0-preview.10 < 1.3.0-preview.9 and base-only prerelease tie-breaks.
• Orphaned .tmp-* extraction directories (from sessions where the cache was undeletable) are swept on startup; fresh ones (< 1 hour) are kept to avoid interfering with concurrent processes.
• The undeletable-cache-directory test now runs on every OS, not just Windows.
• A lock file (.in-use with FileShare.None) guards temp extraction directories from the sweep while another process is still extracting or using them as a session fallback.
• The sweep now enumerates every TFM subdirectory of the cache root, not just the detected TFM.

What changed

• AlcopsAnalyzerProvisioner: ResolveVersionAsync and DownloadAndExtractAsync translate their internal OperationCanceledException into TimeoutException so the existing when (ex is not OperationCanceledException) filters catch it and route to FallbackToCacheOrWarn. ExtractPackage deletes an incomplete targetDir before the move and re-validates after a concurrent-process race. FindNewestCachedVersion uses SemanticVersion instead of Version with the dash-strip trick. SweepStaleTempDirectories enumerates every immediate subdirectory of _cacheRoot (each a TFM folder), sweeps *.tmp-* directories older than one hour, and checks an .in-use lock file before deleting; held directories are skipped. ExtractPackage acquires the lock during extraction, releases it before Directory.Move, and re-acquires it in the return tempDir fallback for the lifetime of the process. The class now implements IDisposable (disposes _inUseLock and HttpClient). A guard before the fallback return tempDir throws IOException if the directory was swept, so the caller falls back to cache.
• SemanticVersion (new): IComparable<SemanticVersion> with TryParse, IsStable, Raw, and a null-safe Comparer. Numeric identifiers compare numerically (arbitrary length); alphanumeric compare OrdinalIgnoreCase; stable > prerelease at equal base; build metadata ignored. The NullSafeComparer nested class is replaced by Comparer<SemanticVersion?>.Create(...).
• NuGetVersions.Parse: replaced ParseVersion/CompareVersions with a single SemanticVersion.TryParse loop. The accumulator variable is renamed from prerelease to newest with a clarifying comment; the tuple element name Prerelease is unchanged.
• BcToolsLocator.OrderByDescendingVersion: now internal static, uses SemanticVersion.Comparer.

Behaviour changes

• A slow NuGet index or package download no longer leaves the session without analyzers when a cached version exists.
• --alcops-analyzers prerelease now correctly resolves 1.3.0-preview.10 over 1.3.0-preview.9.
• The DevTools tool-store probe orders prereleases correctly (numeric, not lexicographic).
• ExtractPackage is now internal (was private) for testability.
• Orphaned .tmp-* extraction directories are cleaned up on each startup.
• Two concurrent alcops-mcp instances sharing the default cache root no longer risk one instance's sweep deleting a temp directory the other is actively using.
• The sweep covers all TFM folders in the cache root, not just the detected TFM.
• Environment.IsPrivilegedProcess replaces Environment.UserName == "root" in the undeletable-cache test.

Test plan

• dotnet build --configuration Release — 0 warnings, 0 errors
• dotnet test --configuration Release — 148 passed, 0 failed, 0 skipped
• All 6 existing NuGetVersionsTests pass unchanged
• All 12 existing BcToolsLocatorTests pass unchanged
• All 6 existing AlcopsAnalyzerProvisionerTests pass unchanged (including Provision_SweepsStaleTempDirectories_KeepsFreshOnes)
• All existing [AlMcpFact] integration tests pass unchanged
• New SemanticVersionTests: 18 test cases (12 comparison theory, 1 parse-odd, 5 reject)
• New NuGetVersionsTests: 5 (two-digit revision, list-order-independence, build-metadata raw return, stable outranks its own RC, newer prerelease outranks older stable)
• New BcToolsLocatorTests: 1 (stable-then-prerelease-numerically, junk last)
• New AlcopsAnalyzerProvisionerTests: 11 (index timeout, download timeout, caller-cancelled, half-extracted replace, target-already-valid, prerelease-numeric fallback, skip-invalid-and-tmp, undeletable cache cross-platform with lock assertions, stale temp directory sweep, sweep skips held directory, sweep covers every TFM folder)

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