Add support for local XML schema files

[Copilot]

This PR adds support for local XML schema files to enable offline validation, addressing the use case where XML documents are used as configuration files and internet access may not be available.

Changes Made

The implementation modifies the get_schema() function to detect whether the schema location is a URL or a local file path:

• URL detection: Uses starts_with("http://") or starts_with("https://") to identify remote schemas
• Local file support: Reads schema files directly from disk using std::fs::read()
• Error handling: Provides clear error messages for missing local schema files
• Backward compatibility: Preserves all existing functionality for URL-based schemas

Example Usage

<!-- Remote schema (existing functionality) -->
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://example.com/ns https://example.com/schema.xsd">
  <content>...</content>
</root>

<!-- Local schema (new functionality) -->
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:schemaLocation="http://example.com/ns ./local-schema.xsd">
  <content>...</content>
</root>

Features

• ✅ Support for both absolute and relative file paths
• ✅ Caching works for both local and remote schemas
• ✅ Proper error messages for missing files
• ✅ Full backward compatibility with existing URL-based schemas
• ✅ No changes to CLI interface or usage

The implementation is minimal and surgical - only the schema loading logic was modified while preserving all existing behavior and performance characteristics.

Fixes #179.

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Changed Files

File	Status
src/main.rs	56% smaller