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/app.rs

@@ -598,6 +598,10 @@ impl App {
         // 5 (export, import).
         match effective_input.as_str() {
             "quit" | "q" => return vec![Action::Quit],
+            "help" => {
+                self.note_help();
+                return Vec::new();
+            }
             "rebuild" => return vec![Action::PrepareRebuild],
             "save" => {
                 return self.handle_save_command(false);
@@ -1024,6 +1028,43 @@ impl App {
         }
     }
 
+    /// Note a flat list of currently-supported app-level
+    /// commands to the output panel.
+    ///
+    /// This is the simple Iteration-4 stand-in for a richer
+    /// help system (H3 in the requirements doc); it gives the
+    /// user a quick "what can I type?" reference that's
+    /// always accurate against the build they're running. As
+    /// new commands land, append them here.
+    fn note_help(&mut self) {
+        self.note_system("Supported commands:");
+        for line in [
+            "  quit / q              — exit",
+            "  help                  — this list",
+            "  mode simple|advanced  — switch input mode",
+            "  rebuild               — rebuild .db from project.yaml + data/ (with confirmation)",
+            "  save                  — save current temp project under a name",
+            "  save as               — copy current project to a new name/path",
+            "  new                   — close current, start a fresh temp project",
+            "  load                  — open the project picker",
+            "DSL data commands (in simple mode):",
+            "  create table <T> with pk [<col>:<type>...]",
+            "  drop table <T>",
+            "  add column [to table] <T>: <col> (<type>)",
+            "  add 1:n relationship [as <name>] from <P>.<col> to <C>.<col>",
+            "    [on delete <action>] [on update <action>] [--create-fk]",
+            "  drop relationship <name>",
+            "  insert into <T> [(cols)] [values] (vals)",
+            "  update <T> set <c>=<v>... where <c>=<v> | --all-rows",
+            "  delete from <T> where <c>=<v> | --all-rows",
+            "  show table <T>",
+            "  show data <T>",
+            "Types: text, int, real, decimal, bool, date, datetime, blob, serial, shortid",
+        ] {
+            self.note_system(line);
+        }
+    }
+
     fn handle_mode_command(&mut self, raw: &str) {
         let arg = raw.strip_prefix("mode").unwrap_or(raw).trim();
         match arg {