fix: normalise xmlns attribute placement in New-PlasterManifest for PS5.1

[HeyItsGilbert]

Closes #461

Summary

• PS5.1's .NET XML serializer ignores XmlWriterSettings.NewLineOnAttributes for namespace declarations, emitting xmlns inline instead of on its own indented line
• After $xmlWriter.Dispose(), reads the written file back and applies a regex substitution to move an inline xmlns= attribute onto its own line, matching the format the tests expect
• A $wroteFile flag gates the post-processing so it only runs when ShouldProcess returned true (skipped in -WhatIf mode)

Test plan

• Run Pester suite on PS5.1 — all four New-PlasterManifest "Generates a valid manifest" tests pass
• Run Pester suite on PS7 — no regression

🤖 Generated with Claude Code

[github-actions]

Test Results

  4 files  284 suites   1m 1s ⏱️
243 tests 237 ✅ 2 💤 4 ❌
972 runs  960 ✅ 8 💤 4 ❌

For more details on these failures, see this check.

Results for commit c9573e0.