PlatyPS 2c: migrate Build-PSBuildMAMLHelp to Export-MamlCommandHelp

[tablackburn]

Part of #120 (Phase 2 — PlatyPS migration, #105). Blocked by the markdown migration — MAML is generated from the markdown this chain produces, so the schema has to settle first.

Scope

Migrate PowerShellBuild/Public/Build-PSBuildMAMLHelp.ps1 from New-ExternalHelp to Import-MarkdownCommandHelp | Export-MamlCommandHelp.

Closes #100 (Build-PSBuildMAMLHelp tests) — written against the new API.

Watch for

• The new pipeline is two cmdlets where the old was one; error handling on a malformed markdown file needs to fail the build rather than silently emit partial MAML
• Verify the emitted MAML is loadable: Get-Help against the built module should return correct syntax, parameter, and example blocks — the 5.1 validation on [Tracking] PowerShellBuild v1.0.0 roadmap #120 confirmed the pipeline produces parseable MAML, but not that this repo's docs survive it

Done when

Build-PSBuildMAMLHelp uses only 1.x cmdlets, #100's tests cover it, Get-Help output is verified end-to-end, and any behavior change has a migration-guide entry.

[tablackburn]

Route call settled 2026-08-24: #150 and #151 land as one pull request.

They are coupled by the markdown schema — migrating Build-PSBuildMarkdown alone hands
Build-PSBuildMAMLHelp input it cannot parse, and the two PlatyPS modules cannot be imported
into one session to bridge the gap. Splitting them produces a state that does not build.

That pull request also carries the dependency swap, since CI needs 1.x from this point onward:

• requirements.psd1 — platyPS = '0.14.2' → Microsoft.PowerShell.PlatyPS = '1.0.3'
• PowerShellBuild.psd1 RequiredModules — the platyPS entry. Consumer-facing and
  breaking, so it needs an entry in docs/migration-v0.8-to-v1.0.md.
• the GenerateMarkdown and GenerateMAML PreConditions in psakeFile.ps1 and IB.tasks.ps1

Deliberately not included: the GenerateUpdatableHelp PreCondition stays pointing at
platyPS, so that task skips with a warning rather than crashing on a New-ExternalHelpCab
that is no longer installed. Build-PSBuildUpdatableHelp has never worked (#169) and its tests
are already skipped, so nothing observable regresses. #152 picks it up. Say so in the pull
request rather than leaving it to be discovered.

Baseline to regress against: #170 (#149), test-only, all CI legs green.

Rationale and the options that were rejected:
#120 (comment) and the PlatyPS
chain shape row on #120.