P2-6: LSP incremental text sync + semantic tokens

[ajitpratap0]

Context

From architect review (Feb 14). LSP does full-document replacement on each change. Missing semantic tokens.

Tasks

• Implement TextDocumentSyncKind.Incremental in documents.go
• Add semantic token provider for SQL keyword highlighting
• Debounce diagnostics on rapid typing
• Route handler through gosqlx API instead of importing parser directly

Files

• pkg/lsp/documents.go
• pkg/lsp/handler.go

Impact

Editor responsiveness on large SQL files. Semantic tokens improve highlighting accuracy.