Versioning Phase 4 — create the `Buildvana.Versioning` shared library

[rdeago]

Sub-issue of #267. (Pure library; lands after Phase 1 but doesn't depend on it.)

Scope

• New src/Buildvana.Versioning/ targeting $(StandardTfm), dependency NuGet.Versioning only — no git, MSBuild, or config dependency.
• Move VersionSpec verbatim from Buildvana.Tool.
• VersionFileData — immutable record parsed from current-version.json ({ major, minor, prerelease }); reject malformed input. Define the "fail loudly if the file is absent" contract for callers.
• VersionCalculator.Compute(fileData, height, branchName, publicReleaseRegexes, prereleaseTag) → { SemanticVersion, CurrentStr, IsPublicRelease, IsPrerelease }. Branch regexes and the prerelease tag are inputs (callers source them from config); IsPublicRelease = branchName matches any regex (short name, explicit anchors). No pathFilters.
• Unit tests: version-string construction, regex matching (empty list, detached-HEAD/empty branch), current-version.json parsing (valid, malformed).
• The library is added and tested but not yet wired into the live tool path (the tool still uses nbgv until Phase 5) — no behavior change.

Acceptance criteria

• Library builds cleanly; VersionSpec no longer exists under Buildvana.Tool.
• Unit tests for VersionCalculator and VersionFileData pass in CI.
• bv behavior is unchanged at this phase (NBGV still in use).
• CHANGELOG: internal-only; note the new library if surfaced, otherwise no public entry.

[rdeago]

Superseded by #293/#294/#296, see #267.