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/app.rs

@@ -101,6 +101,11 @@ pub struct App {
     /// logical OutputLines. Required for accurate scroll capping
     /// when long lines wrap to multiple display rows.
     pub last_output_total_wrapped: usize,
+    /// Prettified display name of the currently-open project,
+    /// rendered in the status bar (P-NAME-3, ADR-0015 §2). `None`
+    /// during very-early startup before the runtime has opened a
+    /// project; otherwise always populated.
+    pub project_name: Option<String>,
 }
 
 const PAGE_SCROLL_LINES: usize = 5;
@@ -130,6 +135,7 @@ impl App {
             output_scroll: 0,
             last_output_visible: 0,
             last_output_total_wrapped: 0,
+            project_name: None,
         }
     }