📖 [Docs]: Document the Pester major-version lock for test files (#57)

The PowerShell Test Specification now documents how test files pin the
Pester framework, so contributors declare the same major-version lock
everywhere.

## Changed: the Test Specification documents the Pester version lock

A new "Pester version" section shows the requirement each `*.Tests.ps1`
file declares:

```powershell
#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
```

It explains that any `6.x` is accepted, that the Invoke-Pester action
installs a matching version, and that GUID identity-pinning is an
optional, separate control rather than part of the default lock.

## Technical Details

- Adds a "Pester version" section to
`src/docs/PowerShell/Modules/Test-Specification.md`.
- Part of the dependency-management epic PSModule/Process-PSModule#356.

Author: MariusStorhaug

src/docs/PowerShell/Modules/Test-Specification.md

@@ -4,6 +4,19 @@
 
 This document defines the structure and guidelines for writing Pester tests for PowerShell functions. The goal is to ensure consistency and comprehensive test coverage while maintaining clarity.
 
+## Pester version
+
+Lock the test framework to a **major version** so a new Pester major cannot be adopted silently and break every suite at once. Declare it as the first line of each `*.Tests.ps1` file:
+
+```powershell
+#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
+```
+
+- `ModuleVersion` + `MaximumVersion` accept any Pester `6.x`, so routine minor and patch updates flow in without a change here — only a new major requires a deliberate, reviewed bump.
+- The `MaximumVersion = '6.*'` wildcard locks to the major without a sentinel version; PowerShell enforces the range at discovery time.
+- The [Invoke-Pester](https://github.com/PSModule/Invoke-Pester) action installs a matching `6.x`, so the requirement and the installed version stay aligned.
+- Pinning module identity with a `GUID` is a separate, stricter supply-chain control — omit it for the default major-lock; add it only when identity assurance is required.
+
 ## Test Structure
 
 Each function is tested within a structured Pester `Describe` block that follows this hierarchy: