Iteration 1: file-backed projects with auto-named temps, lock file, and L1 CLI

Replaces the in-memory database with an on-disk project. Startup either
opens a project at the positional CLI path (L1) or creates an auto-named
temp project (<YYYYMMDD>-<word>-<word>-<word>) under the OS-standard
data directory or a --data-dir override. The new project::Project type
owns the directory skeleton and a PID+hostname lock file with
stale-lock takeover via sysinfo. The status bar now shows
"Project: <Display Name>", derived by a small kebab/snake/camel
prettifier. Per-command persistence to YAML/CSV/history.log is NOT
yet wired -- that's Iteration 2; for now playground.db carries the
state across quits.

Tests: 257 passing (231 lib + 9 new integration + 17 existing),
0 failing, 0 skipped. Clippy clean with nursery lints.

src/db.rs

@@ -279,7 +279,7 @@ impl Database {
     /// Open a database. The path may be a filesystem location
     /// or `":memory:"` for an ephemeral in-memory database. The
     /// connection is moved onto a dedicated worker thread.
-    pub fn open<P: AsRef<Path> + Into<String>>(path: P) -> Result<Self, DbError> {
+    pub fn open<P: AsRef<Path>>(path: P) -> Result<Self, DbError> {
         let path_display = path.as_ref().to_string_lossy().into_owned();
         let conn = match path.as_ref().to_str() {
             Some(":memory:") => Connection::open_in_memory(),