Cleanup pass: --help, in-app help, post-rebuild message, unmodified-temp cleanup

Four post-Iteration-4 polish items surfaced by manual testing.

1. `--help` / `-h` CLI flag prints a usage banner (options +
   app-level commands + DSL grammar reference) and exits. Parse
   errors also print the banner to stderr.

2. `help` app-level command notes the same list of supported
   commands to the output panel -- a simple stand-in for the
   richer H3 help system, kept in sync with what's actually
   wired up.

3. The silent rebuild that runs when playground.db is missing
   now surfaces a system message in the output panel ("[ok]
   rebuild -- N tables, M rows reconstructed; ...") via a new
   initial_events plumbing. The user no longer wonders whether
   the .db was magically restored or whether anything happened
   on launch.

4. Unmodified empty temp projects (kind=Temp, project.yaml has
   tables: [] and relationships: []) are now auto-deleted when
   the user switches away (load / new / save as) or quits. This
   addresses the "launch app, load existing project, quit"
   pattern that was leaving an empty temp directory behind
   every time. Modified temps (with any user-created tables or
   relationships) are never auto-deleted; corrupted projects
   are also never auto-deleted (defensive default-to-false on
   yaml read/parse errors).

Tests: 338 passing (272 lib + 9 + 5 + 6 + 20 + 9 + 17),
0 failing, 0 skipped. Clippy clean.

src/cli.rs

@@ -20,8 +20,45 @@ pub struct Args {
     /// this path (L1, ADR-0015 §1). Mutually exclusive with
     /// `--resume` once that lands.
     pub project_path: Option<PathBuf>,
+    /// `--help` / `-h`: print usage to stdout and exit. The
+    /// runtime checks this flag before doing any other work.
+    pub help: bool,
 }
 
+/// Usage banner printed by `--help`. Kept as one block so the
+/// formatting is reviewable on its own.
+pub const HELP_TEXT: &str = "\
+rdbms-playground — a TUI playground for relational database concepts
+
+Usage:
+    rdbms-playground [options] [<project-path>]
+
+Arguments:
+    <project-path>           Path to an existing project directory.
+                             Without this, a fresh auto-named temp
+                             project is created in the data dir.
+
+Options:
+    -h, --help               Print this help and exit.
+        --theme <light|dark> Override theme (default: auto-detect).
+        --data-dir <PATH>    Use PATH as the data root instead of
+                             the OS-standard location for this run.
+        --log-file <PATH>    Write tracing output to PATH.
+
+App-level commands (typed inside the app, available in both modes):
+    quit / q                 Exit cleanly.
+    mode simple|advanced     Switch input mode.
+    help                     Show this list of commands in-app.
+    save                     Save the current temp project under a
+                             chosen name (or `save as` to copy a
+                             named project to a new location).
+    save as                  Always prompt for a target name/path.
+    new                      Close current, create a fresh temp.
+    load                     Open the project picker.
+    rebuild                  Rebuild playground.db from project.yaml
+                             + data/, with confirmation.
+";
+
 #[derive(Debug, thiserror::Error)]
 pub enum ArgsError {
     #[error("missing value for --{0}")]
@@ -54,9 +91,13 @@ impl Args {
         let mut log_path = env::var_os("RDBMS_PLAYGROUND_LOG_FILE").map(PathBuf::from);
         let mut data_dir: Option<PathBuf> = None;
         let mut project_path: Option<PathBuf> = None;
+        let mut help = false;
         let mut iter = iter.into_iter().map(Into::into);
         while let Some(arg) = iter.next() {
             match arg.as_str() {
+                "--help" | "-h" => {
+                    help = true;
+                }
                 "--theme" => {
                     let value = iter.next().ok_or(ArgsError::MissingValue("theme"))?;
                     theme = match value.as_str() {
@@ -98,6 +139,7 @@ impl Args {
             log_path,
             data_dir,
             project_path,
+            help,
         })
     }
 }
@@ -203,6 +245,18 @@ mod tests {
         assert!(matches!(err, ArgsError::MultiplePaths { .. }), "got: {err:?}");
     }
 
+    #[test]
+    fn help_flag_long_form_sets_help() {
+        let args = Args::parse(["--help"]).unwrap();
+        assert!(args.help);
+    }
+
+    #[test]
+    fn help_flag_short_form_sets_help() {
+        let args = Args::parse(["-h"]).unwrap();
+        assert!(args.help);
+    }
+
     #[test]
     fn unknown_double_dash_flag_errors_even_with_positional() {
         // Make sure the path-vs-flag distinction is robust: