feat(a2ml): add LSP server for A2ML diagnostics, completions, and hover

Rust implementation using tower-lsp providing:
- 8 diagnostic checks (SPDX, unclosed directives, invalid refs, etc.)
- Context-aware completions for directives, sections, keys, and @ref() targets
- Hover documentation for directives and section headers
- 12 unit tests, all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath