Lift the Diagnostic out of the orchestrator into src/WcdDiagnostic.ps1

[DireDoch]

Closes #16.

What moved

The 13 functions that decide what the technician reads, lifted byte for byte out
of src/Invoke-WcdConfiguration.ps1 into src/WcdDiagnostic.ps1:
Get-WcdSeverityRank, Get-WcdResultsForSteps, Get-WcdStrongestResult,
Format-WcdRemedy, Get-WcdAggregateDetail, New-WcdDiagnosticEntry,
Resolve-WcdAutomaticEntry, Get-WcdModuleStatusKind, Format-WcdModuleLine,
Format-WcdChecklistLine, Write-WcdSectionHeader,
Get-WcdFinalChecklistEntries, Get-WcdFinalDiagnosticLines.

Read-WcdChoice, Read-WcdComputerName, Wait-WcdForEnter and
Resolve-WcdExecutionOptions stayed: they are the console, not the Diagnostic.

The orchestrator dot-sources the new file beside WcdHelpers.ps1. No behaviour
changes; the moved block is identical to what was removed.

The $T problem

Took the second option from the issue: the functions still read $script:T.
Rather than hand-copy a $T into the test file and let it drift, the test lifts
the real assignment out of the orchestrator's AST and evaluates it with
$ScriptUI = 'EN'. The assertions therefore run against the strings a
technician actually sees, and a renamed key breaks the test instead of passing
silently.

Tests

tests/WcdDiagnostic.Tests.ps1, 16 tests, covering every box on the issue:

• a declined prompt (ApplicationsSkip) yields a Manual Step
• a Step filtered by Environment yields Not Applicable, not a warning
• a Step that could not run yields a failure, with its remediation
• a Step handed back to the technician (MANUAL) yields a Manual Step, not a failure
• an Optional Application Target that is absent yields a note, not a warning
• a Module reporting some but not all of its planned Steps yields a warning naming the missing one
• Format-WcdRemedy with a key absent from $T.Remedy renders nothing
• Format-WcdRemedy with mismatched RemedyArgs returns the template, not a throw
• plus severity ordering, worst-result aggregation, and the history-log rendering

tests/Help.Tests.ps1 still green: comment-based help came across intact.

Verified

Run locally on PowerShell 7.4.6:

• Invoke-Pester ./tests/*.Tests.ps1 — 128 passed, 2 failed. Both failures are
  the pre-existing Windows-only ones (New-WinUserLanguageList,
  Get-CimInstance) that also fail on main from Linux; CI runs
  windows-latest, where they pass.
• Invoke-ScriptAnalyzer -Path . -Recurse -Settings ./PSScriptAnalyzerSettings.psd1 — clean.

One small change that is not a pure move

FatalHelpersMissing now takes the filename as {0}, since there are two files
to dot-source. The rendered sentence for WcdHelpers.ps1 is unchanged, so
console output stays byte-identical.

Left out

docs/manual.typ has two Typst diagrams of the src/ tree that still show two
files. Adding a row means shifting every coordinate in both and rebuilding the
committed PDF — out of scope here, worth its own issue. The README tree is
updated.

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