Release gate: cut 1.0.0-preview2 after Phase 3

[tablackburn]

Part of #120 (Phase 3 gate).

Scope

Cut 1.0.0-preview2 to PSGallery once the API improvements land.

Contents:

• Add support for passing a PesterConfiguration object to PowerShellBuild #83 — PesterConfiguration object support, built to the precedence contract settled in its design ticket
• Settle the FailBuildOnSeverityLevel contract: 'Any' is documented but rejected, and ParseError escapes every threshold #144 — the settled FailBuildOnSeverityLevel contract (ParseError folded into Error, 'Any' added, analyzer-level failures no longer pass the gate), including the Test-PSBuildScriptAnalysis does not fail on the PSScriptAnalyzer rule crash from #136 #147 rule-crash escape
• Scope PlatyPS to Specific Paths #124 — PlatyPS path scoping
• Test-PSBuildPester coverage threshold math truncates every percentage to zero #138 — the coverage-percentage truncation fix

Gate

Blocked on 1.0.0-preview1, so preview1's soak is not muddied by a second wave of changes.

Done when

1.0.0-preview2 is published, and the migration guide covers the #144 behavior change (folding ParseError into the Error threshold can turn a previously-passing consumer build red — exactly the class of change the guide's widened scope was meant to capture).