PlatyPS 2a: add Microsoft.PowerShell.PlatyPS 1.x alongside platyPS

[tablackburn]

Part of #120 (Phase 2 — PlatyPS migration, #105). First link in the PlatyPS chain.

Scope

Install Microsoft.PowerShell.PlatyPS 1.x alongside the existing platyPS 0.14.2 so the two can coexist while the three Build-PSBuild*Help functions are migrated one at a time. Nothing consumer-facing changes in this PR.

• Add Microsoft.PowerShell.PlatyPS to requirements.psd1 (pin an explicit version, as the repo does for every other dependency)
• Leave platyPS = '0.14.2' in place — it is removed in the final link of the chain
• Verify both modules import in the same session without command-name collisions (the new module's cmdlets are *-MarkdownCommandHelp / *-MamlCommandHelp, so no overlap is expected — confirm rather than assume)
• Confirm CI installs both on all legs: Windows PowerShell 5.1, and pwsh on ubuntu/windows/macOS

Notes

Compatibility with Windows PowerShell 5.1 is already established and is not an open question here — the assembly targets net472, the manifest declares no PowerShellVersion floor, and the full markdown→MAML pipeline was validated end-to-end on 5.1.26100. See the 2026-07-21 investigation comment.

Done when

Both modules are declared, build.ps1 -Bootstrap installs them, and CI is green on every leg with no functional change to the help build.