Release gate: cut 1.0.0-preview1 after Phase 2

[tablackburn]

Part of #120 (Phase 2 gate).

Scope

Cut 1.0.0-preview1 to PSGallery — the first prerelease of the 1.0.0 cycle.

Per the preview-numbering decision in #120, preview numbers are decoupled from phase numbers. No preview was cut after Phase 1, so this one carries the Phase 1 shipped changes along with Phase 2:

• Test-PSBuildPester fixes (Tests: Test-PSBuildPester #102) and the manifest version floor (chore: Raise minimum PowerShell version to 5.1 in module manifest #141), from Phase 1
• The complete PlatyPS 1.x migration (Update PlatyPS dependency from platyPS v0.14.x to Microsoft.PowerShell.PlatyPS v1.x #105 chain)
• The psake 5.x outcome — either the migration, or the recorded decision to ship 1.0.0 on 4.9.0

Gate

Blocked on the entire Phase 2 surface so the soak covers the whole break surface at once rather than in slices. The PlatyPS and psake chains run in parallel; both must land (or, for psake, be explicitly cut per the spike's abort criterion) before this cuts.

Done when

1.0.0-preview1 is published to PSGallery, and docs/migration-v0.8-to-v1.0.md covers every break and behavioral change it contains.

[tablackburn]

Closing as superseded. The 1.0.0 cycle now cuts exactly one prerelease, and it is the rc1 already scoped in #160.

Why

#83 and its design ticket #156 are deferred to 1.1.0. With them out, Phase 3 is empty, so the break surface for 1.0.0 is complete as of today — nothing further is going in that breaks anything.

That removed the reason for two prereleases. preview1 and rc1 would have soaked an identical surface, since nothing lands between them any more.

What is preserved

The reason this gate existed is not going away. Several breaks in this cycle are install-time and cannot be observed from a branch or from this repository's CI, which builds this module rather than a consumer's:

• RequiredModules now demands psake 5.0.4 and Pester 6.0.0. A consumer with psake 4.x cannot import the module at all.
• PlatyPS was removed from RequiredModules, so Install-Module PowerShellBuild no longer brings it along and consumers who build help will find it missing.

Only a real gallery install exercises those. #160's rc1 and its ≥ 7 day soak now carry that job.

Superseded by #160. Nothing here is dropped, only merged into a single prerelease.