Tests: Create shared fixture module

[HeyItsGilbert]

Create a minimal shared fixture module to be used by the integration-test sub-tasks of #17 (#98, #99, #100, #101, #102, #103).

Agreed design (2026-07-09)

Location and name: tests/fixtures/PSBuildTestFixture/. The name is deliberately distinct from the existing tests/TestModule/ — PowerShell session state is keyed by module name, so two modules both named TestModule would invite cross-contamination between test files.

Contents (inert data only — no build files, no requirements.psd1, no nested test suite):

• PSBuildTestFixture.psd1 — valid manifest, version 0.1.0
• PSBuildTestFixture.psm1 — dot-sources Public/ and Private/
• Public/ — two functions with complete comment-based help (synopsis, description, parameters, examples) so the help-pipeline tests (Tests: Build-PSBuildMarkdown #99, Tests: Build-PSBuildMAMLHelp #100, Tests: Build-PSBuildUpdatableHelp #101) have real material. No .LINK URLs, so no test ever makes a network call
• Private/ — one internal helper, for export and compile assertions (Tests: Build-PSBuildModule #98)
• excludeme.txt — exercises $PSBPreference.Build.Exclude patterns (Tests: Build-PSBuildModule #98)
• tests/fixtures/FixtureHelpers.psm1 — Copy-PSBuildTestFixture helper that copies the fixture into $TestDrive; consuming tests always work on a copy, never the checked-in fixture

Scope boundaries:

• The existing tests/TestModule/ is untouched — it keeps serving tests/build.tests.ps1 for end-to-end child-process builds. Consolidating the two fixtures, if ever desired, is a separate issue
• Crash/failure test files (needed by Tests: Test-PSBuildPester #102) are not checked in — they get generated into $TestDrive at runtime so the repository's own Pester run can never discover them
• Fixture content stays minimal; sub-tasks add material (for example Classes//Enum/ directories for compile-order tests) in the pull request that actually tests it

Ships with a smoke test (tests/Fixtures.tests.ps1): manifest validates, module imports from a $TestDrive copy, expected exports.

Target Pester 6.0.0 with classic Should syntax — see notes on #17. Parent issue: #17