fix: add PS5.1 SemanticVersion shim for consistent Resolve-ModuleVersionString return type

[HeyItsGilbert]

Closes #460

Summary

• [System.Management.Automation.SemanticVersion] is PS6+ only; on PS5.1 Resolve-ModuleVersionString was falling back to [System.Version], causing two test assertions to fail
• Adds Plaster/Private/Initialize-SemanticVersion.ps1: a top-level Add-Type shim that defines System.Management.Automation.SemanticVersion in C# when the native type is absent
• Simplifies Resolve-ModuleVersionString to always construct SemanticVersion — the shim ensures the type exists on PS5.1 at module load time (alphabetical load order puts Initialize-SemanticVersion before Resolve-ModuleVersionString)
• Tests require no changes

Test plan

• Run Pester suite on PS5.1 — both Resolve-ModuleVersionString tests pass
• Run Pester suite on PS7 — no regression

🤖 Generated with Claude Code

[github-actions]

Test Results

  4 files  284 suites   1m 11s ⏱️
243 tests 236 ✅ 2 💤 5 ❌
972 runs  959 ✅ 8 💤 5 ❌

For more details on these failures, see this check.

Results for commit b2e162b.