M1: Validate generated MAML against official schema (XSD)

[jeffpatton1971]

Validate produced MAML using XSD validation and fail generation when invalid.

Implementation notes

• Store schema(s) under something like src/HelpGenerator.Core/Schemas/ (embedded resource or content copied at runtime).
• Implement IXmlSchemaValidator or a simple service class MamlSchemaValidator.

Definition of done

• Validator returns structured errors (line/position/message).
• Generation path throws a friendly exception including validation failures.

Acceptance checks

• Unit test: valid XML passes.
• Unit test: intentionally broken XML fails with an error list.

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5) File output writer (Core or PowerShell)

Title: M1: Write MAML output to disk using updatable help folder structure (en-US/<module>-help.xml)
Type: Feature
Labels: adr:001, milestone:m1, area:core, type:feature

Description
Implement the file writing logic for MAML output.

M1 file layout

<OutputRoot>/
  en-US/
    <ModuleName>-help.xml

Definition of done

• Output directory created if missing.
• -Force support (delete/overwrite existing output).
• No timestamps/random IDs in filenames.

Acceptance checks

• Integration test verifies correct file path(s) exist after generation.