…zed build/test (#449)

## Overview

This PR lands the long-running `v2` branch into `master`, shipping
Plaster **2.0.0-alpha1**. It's a large drop (~50 commits, ~115 files,
+11.5k / -3.2k) and touches nearly every part of the project, so the
sections below break it down by theme rather than by commit.

Target version: `Plaster.psd1` → `ModuleVersion = '2.0.0'`, `Prerelease
= 'alpha1'`.

## Highlights

- **JSON manifest support** — Plaster templates can now be authored as
JSON in addition to XML, with a full `plaster-manifest-v2.json` schema,
bidirectional conversion, and validation.
- **Modular source layout** — the monolithic `Invoke-Plaster.ps1` /
`NewPlasterManifest.ps1` / `GetModuleExtension.ps1` files have been
decomposed into per-function `Public/` and `Private/` files.
- **PowerShell 7.x + cross-platform** — Windows, Linux, macOS all
supported; minimum PS version is now 5.1.
- **Pester 5.x test suite** — large coverage expansion with per-function
tests.
- **Modernized build** — psake + PowerShellBuild + PSDepend, with a
`-Bootstrap` switch for clean CI installs.
- **Publish workflow** rewired end-to-end and validated against
GitHub-hosted runners.

## What changed

### Module architecture
- Split `Plaster/InvokePlaster.ps1` (1500+ lines),
`Plaster/NewPlasterManifest.ps1`, and `Plaster/GetModuleExtension.ps1`
into discrete `Public/` and `Private/` function files.
- New `Plaster/Private/` helpers for things that were previously inline:
`ConvertFrom-JsonManifest`, `ConvertTo-JsonManifest`,
`ConvertFrom-JsonContentAction`, `ConvertTo-JsonContentAction`,
`Copy-FileWithConflictDetection`, `Expand-FileSourceSpec`,
`Get-ManifestsUnderPath`, `Get-PlasterManifestType`,
`Initialize-PredefinedVariables`, `Invoke-ExpressionImpl`,
`Invoke-PlasterOperation`, `New-ConstrainedRunspace`,
`Resolve-ModuleVersionString`, `Resolve-ProcessParameter`,
`Start-Process*`, `Write-PlasterLog`, and friends.
- `Plaster.psm1` / `Plaster.psd1` updated to match the new layout.

### JSON manifest support (new)
- `Plaster/Schema/plaster-manifest-v2.json` — full JSON Schema for v2
manifests.
- JSON ↔ internal-model converters: `ConvertFrom-JsonManifest`,
`ConvertTo-JsonManifest`, plus content-action variants.
- `Get-PlasterManifestType` detects XML vs JSON manifests;
`Test-PlasterManifest` validates both.
- Example JSON manifests under `examples/` (`plasterManifest.json`,
`plasterManifest-validatePattern.json`, `plasterManifest_fr-FR.json`).
- Fixed error handling around file and template actions in JSON flow.

### Cross-platform / PS7
- Minimum PowerShell version raised to 5.1; null-coalescing operators
removed from hot paths for PS5.1 compatibility (#443).
- Default encoding changed from `Default` to `UTF-8-NoBOM`.
- Path separator handling normalized; constrained-runspace compatibility
fixes for PS7.
- Parameter default value storage fixed on non-Windows.

### Tests
- Migrated to **Pester 5.x** (breaking change for test authors).
- New per-function test files: `Get-ModuleExtension.Tests.ps1`,
`Get-PlasterManifestPathForCulture.Tests.ps1`,
`Initialize-PredefinedVariables.Tests.ps1`,
`Resolve-ModuleVersionString.Tests.ps1`,
`New-PlasterManifest.Tests.ps1`, etc.
- Added `Help.tests.ps1`, `Manifest.tests.ps1`, `Meta.tests.ps1`,
`MetaFixers.psm1` (imported from Stucco).
- Large fixture added for module resolution tests
(`tests/Fixtures/ModuleList.xml`).
- Conditional-attribute evaluation tests fixed (#445).

### Build & tooling
- `build.ps1` refactored for cleaner parameter handling, with `-Task`,
`-Bootstrap`, and `-Help` parameter sets.
- `-Bootstrap` provisions PSDepend (pinned to PowerShellGet v2), then
runs a try-import-first-then-install pattern against
`requirements.psd1`.
- `requirements.psd1` pins PSDepend 0.3.8, Pester 5.7.1, psake 4.9.1,
BuildHelpers 2.0.16, PowerShellBuild 0.7.2, PSScriptAnalyzer 1.24.0.
- `psakeFile.ps1` delegates to PowerShellBuild; `CopyDirectories`
configured for `en-US`, `Schema`, `Templates`.
- `debugHarness.ps1` refactored for the new module layout.
- `.vscode/` tasks + settings refreshed; `.markdownlint.json` +
`cspell.json` added.

### CI / publish workflow
- `.github/workflows/publish.yaml` now runs on `windows-latest`, uses
`ModuleFast-action` to pre-stage deps, and invokes `./build.ps1 -Task
Publish -Bootstrap`.
- `.github/workflows/PesterReports.yml` updated permissions and action
versions.
- Dead placeholder \`internal-nuget-repo\` registration removed from the
bootstrap path (was failing \`Register-PSRepository\` URI validation on
GitHub runners).

### Docs
- README rewritten for v2.
- CHANGELOG reformatted to Keep a Changelog.
- Logo updated to make the 2.0 branding clearer.
- Examples directory expanded.

## Breaking changes

- **Minimum PowerShell version** is now 5.1 (was 3.0).
- **Test framework** is Pester 5.x — any downstream tests authored
against Pester 3/4 will need updating.
- **Default encoding** changed to `UTF-8-NoBOM`.
- Internal module layout changed significantly; anything dot-sourcing
Plaster internals by path will break (public API is unchanged).

## Test plan

- [ ] CI green on `v2` (publish workflow dry-run + Pester reports)
- [ ] \`./build.ps1 -Bootstrap -Task Test\` passes locally on Windows PS
5.1
- [ ] \`./build.ps1 -Bootstrap -Task Test\` passes on Windows PS 7
- [ ] Smoke test on Linux / macOS (PS 7)
- [ ] \`Invoke-Plaster\` against an XML manifest still works
(regression)
- [ ] \`Invoke-Plaster\` against a JSON manifest works end-to-end
- [ ] \`Test-PlasterManifest\` validates both XML and JSON manifests
- [ ] Publish workflow successfully pushes \`2.0.0-alpha1\` to PSGallery
when manually dispatched

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