Guard against drift between `plasterManifest.xml` and the `Invoke-Plaster` test stub

**Problem description**
PR #560 added unit tests in `tests/Unit/Public/New-SampleModule.tests.ps1` that replace `Invoke-Plaster` with a hand-maintained stub listing every Plaster parameter statically. The stub is required because `Invoke-Plaster` declares manifest-driven parameters via `dynamicparam`, which Pester's auto-generated mock proxy cannot model.

When a new `Use*` toggle or parameter is added to `Sampler/Templates/Sampler/plasterManifest.xml`, the stub will silently drop it — `$PSBoundParameters` will not contain it and the tests will keep passing while real scaffolding behavior regresses.

**Verbose logs**
N/A

**How to reproduce**
1. Add a new `` entry to `Sampler/Templates/Sampler/plasterManifest.xml`.
2. Do not update the stub in `tests/Unit/Public/New-SampleModule.tests.ps1`.
3. Run `./build.ps1 -Tasks test -PesterPath 'tests/Unit/Public/New-SampleModule.tests.ps1' -CodeCoverageThreshold 0`.
4. Observe that tests pass even though the stub does not accept the new parameter.

**Expected behavior**
A test fails when the stub's `param()` block is out of sync with the parameters defined in `plasterManifest.xml`.

**Current behavior**
No such check exists; drift is silent.

**Suggested solution**
Add a lightweight guard test under `tests/Unit/Public/` that:

1. Parses `Sampler/Templates/Sampler/plasterManifest.xml`.
2. Enumerates `` nodes.
3. Asserts every name is present in `(Get-Command Invoke-Plaster).Parameters.Keys` within `InModuleScope`.

Consider also documenting the stub pattern in `.github/instructions/test-writing.instructions.md`.

**Operating system the target node is running**
N/A

**PowerShell version and build the target node is running**
N/A

**Module version used**
Sampler `Unreleased` / post-#560.