build(workspace): introduce Cargo workspace with lovcode-core skeleton

Add root Cargo.toml workspace with shared deps and version 0.40.0-dev.
Create crates/lovcode-core lib crate with module skeleton (types,
adapter, index, query, watcher) — implementations land in phase 1.2/1.3.
src-tauri stays a workspace member; its old per-crate Cargo.lock is
removed in favor of the workspace-level lock.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: MarkShawn2020

.gitignore

@@ -26,3 +26,6 @@ dist-ssr
 # Local workspace artifacts
 .claude/
 output/
+
+# Cargo workspace build artifacts
+/target/