Implement cross-platform Metadata parsing library with variable substitution

[Copilot]

Per README requirements, implement a library to parse XML metadata files with Include resolution and variable substitution support (${ENV:}, ${MD5:}, ${META:}, ${this:}).

Changes

• Path handling: Replace hardcoded backslashes with Path.DirectorySeparatorChar for cross-platform compatibility in Include/Module resolution
• MD5 variable resolution: Implement smart ${MD5::} that computes hash of file referenced in Source attribute when used in Hash attribute

  <Markdown Source="file.md" Hash="${MD5::}"/>
  <!-- Resolves to MD5 hash of file.md -->

• Nullable safety: Fix nullable reference warnings in environment variable enumeration and path operations

Library API

// Load and parse with Include resolution
var metadata = Metadata.LoadFromFile("metadata.xml");

// Replace variables in-place
metadata.ReplaceVariablesInFile("metadata.xml");

// Compile to merged document
var doc = metadata.Compile();

Supports variable types: ${ENV:VAR}, ${MD5::}, ${MD5:file.txt}, ${META:path:to:element}, ${this:DefineKey}

Original prompt

按照README要求，编写一个负责解析Metadata的库

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