Refactor: Consolidate document state management in Workspace

[jade-codes]

Problem

The LSP layer duplicates state that should live in Workspace:

• document_texts: HashMap<PathBuf, String> - tracked separately from workspace files
• parse_errors: HashMap<PathBuf, Vec> - tracked separately from workspace files
• Direct std::fs::read_to_string calls bypassing core abstractions

This violates separation of concerns - LSP should be a thin protocol adapter, not a state manager.

Implementation Plan

• Add source_text: String to WorkspaceFile
• Add errors: Vec to WorkspaceFile
• Create FileLoader trait for abstraction (disk/memory/virtual sources)
• Add update_from_text(path, text) method to Workspace
• Remove document_texts from LspServer, use workspace.get_text(path)
• Remove parse_errors from LspServer, use workspace.get_errors(path)
• Remove direct std::fs calls from LSP layer

Benefits

• Single source of truth for document state
• LSP becomes a thin protocol adapter
• Easier testing (mock FileLoader)
• Better separation of concerns