In-Memory Document Sync (didOpen/didChange)

[Francisco-Galindo]

Maintain a real-time copy of the editor's text buffer in server memory.

Checklist:

• Create a DocumentStore data structure to track open files.
• Implement textDocument/didOpen to load file state.
• Implement textDocument/didChange to update state on every keystroke.
• Implement textDocument/didClose to purge memory.