feat: LSP server for MDS

[dean0x]

Summary

Build a Language Server Protocol (LSP) implementation for MDS, providing real-time diagnostics, completions, go-to-definition, hover info, and more across all editors that support LSP.

Motivation

An LSP server is the "makes MDS feel like a real language" feature. It provides:

• Real-time error diagnostics as you type (no need to run mds check)
• Autocomplete for variable names, function names, and import paths
• Go-to-definition for @import paths and function calls
• Hover info showing variable types/values and function signatures
• Rename symbol across files
• Find references for variables and functions

This is the flagship developer experience investment and benefits every editor simultaneously (VS Code, Neovim, Helix, Zed, Emacs, Sublime Text, JetBrains).

Scope

Phase 1 — Core Features

• Diagnostics (delegate to mds check + lint rules)
• Completion for frontmatter variables, defined functions, imported symbols
• Go-to-definition for @import paths, function definitions
• Hover info for variables (show value/type) and functions (show signature)

Phase 2 — Advanced Features

• Workspace-wide diagnostics (all .mds files, not just the open one)
• Rename symbol (variables, functions) across files
• Find all references
• Code actions (quick fixes for common lint warnings)
• Signature help (parameter hints during function calls)

Phase 3 — Integration

• Document formatting (delegates to mds fmt)
• Document symbols (outline view)
• Folding ranges
• Semantic tokens (richer highlighting than TextMate/Tree-sitter)

Design Considerations

• Implementation: Rust, using the tower-lsp crate (standard Rust LSP framework)
• Crate: New crates/mds-lsp in the workspace
• Distribution: Standalone binary (mds-lsp) + bundled in the VS Code extension
• Incremental: The LSP must be responsive — reparse only changed regions, cache module resolutions
• Configuration: Reads mds.json for project settings, respects lint rule configuration
• Testing: LSP protocol-level tests using tower-lsp-testing or similar

Acceptance Criteria

• mds-lsp binary starts and communicates via LSP protocol
• Real-time diagnostics on file save/change
• Variable and function completion
• Go-to-definition for imports and functions
• Hover info for variables and functions
• VS Code extension uses the LSP server
• Neovim configuration documented
• Response time < 100ms for completions on files up to 1000 lines