feat(cli): --demo demonstration mode flag + app plumbing (#22, ADR-0047 D1)

Add `--demo` (and the RDBMS_PLAYGROUND_DEMO env fallback) to enter
demonstration mode, threaded onto App.demo_mode through run_loop —
mirrors the --no-undo plumbing. Off by default, zero footprint when
off. The --help line advertises only the visible keystroke badges;
the Ctrl+] caption trigger is kept low-profile (ADR-0047 D6 updated).

Phase A of ADR-0047; behaviour (badges/captions) lands in B and C.

src/cli.rs

@@ -42,6 +42,13 @@ pub struct Args {
     /// mode > the default (`simple`). Combines with `--resume` and
     /// a positional path; on collision the flag wins.
     pub mode: Option<Mode>,
+    /// `--demo` (or `RDBMS_PLAYGROUND_DEMO` set truthy): enter
+    /// **demonstration mode** (ADR-0047, issue #22). Off by default,
+    /// zero footprint when off. When on, the app shows transient
+    /// on-screen badges for otherwise-invisible keys (Tab, Enter, …)
+    /// and enables the `Ctrl+]` stealth step-caption buffer — for
+    /// screencasts and live teaching. The flag wins over the env var.
+    pub demo: bool,
 }
 
 /// Usage banner printed by `--help`.
@@ -124,6 +131,12 @@ impl Args {
         let mut help = false;
         let mut no_undo = false;
         let mut mode: Option<Mode> = None;
+        // Demonstration mode (ADR-0047): the env var is the default,
+        // the `--demo` flag overrides it to on. Mirrors the
+        // env-then-flag layering used for the log file above.
+        let mut demo = env::var("RDBMS_PLAYGROUND_DEMO")
+            .ok()
+            .is_some_and(|v| demo_value_is_truthy(&v));
         let mut iter = iter.into_iter().map(Into::into);
         while let Some(arg) = iter.next() {
             match arg.as_str() {
@@ -136,6 +149,9 @@ impl Args {
                 "--no-undo" => {
                     no_undo = true;
                 }
+                "--demo" => {
+                    demo = true;
+                }
                 "--theme" => {
                     let value = iter.next().ok_or(ArgsError::MissingValue("theme"))?;
                     theme = match value.as_str() {
@@ -194,10 +210,25 @@ impl Args {
             help,
             no_undo,
             mode,
+            demo,
         })
     }
 }
 
+/// Whether a `RDBMS_PLAYGROUND_DEMO` value enables demo mode.
+///
+/// Truthy for any value except the conventional "off" set
+/// (`0`/`false`/`no`/`off`, case-insensitively, and the empty
+/// string). So `RDBMS_PLAYGROUND_DEMO=1` and `=true` enable, while
+/// `=0` / `=false` explicitly disable — letting a value of `0` turn
+/// it off even if something upstream exported the variable.
+fn demo_value_is_truthy(value: &str) -> bool {
+    !matches!(
+        value.trim().to_ascii_lowercase().as_str(),
+        "" | "0" | "false" | "no" | "off"
+    )
+}
+
 fn default_theme() -> Theme {
     // NFR-7: support both backgrounds. For the walking skeleton we
     // honour an explicit `--theme` flag and the COLORFGBG env var
@@ -391,6 +422,52 @@ mod tests {
         );
     }
 
+    // ---- ADR-0047 (issue #22): --demo demonstration mode ----
+
+    #[test]
+    fn demo_flag_parses() {
+        let args = Args::parse(["--demo"]).unwrap();
+        assert!(args.demo);
+    }
+
+    #[test]
+    fn demo_defaults_off() {
+        // Absent `--demo` (and absent env var in the test runner),
+        // demo mode is off — zero footprint for real users.
+        let args = Args::parse(std::iter::empty::<&str>()).unwrap();
+        assert!(!args.demo, "demo is off unless --demo or the env var is given");
+    }
+
+    #[test]
+    fn demo_flag_coexists_with_positional_path() {
+        let args = Args::parse(["--demo", "/home/me/MyProject"]).unwrap();
+        assert!(args.demo);
+        assert_eq!(
+            args.project_path.as_deref(),
+            Some(std::path::Path::new("/home/me/MyProject"))
+        );
+    }
+
+    #[test]
+    fn demo_flag_combines_with_resume_and_mode() {
+        let args = Args::parse(["--resume", "--demo", "--mode", "advanced"]).unwrap();
+        assert!(args.demo);
+        assert!(args.resume);
+        assert_eq!(args.mode, Some(Mode::Advanced));
+    }
+
+    #[test]
+    fn demo_env_value_truthiness() {
+        // Enabling values.
+        for v in ["1", "true", "TRUE", "yes", "on", "anything", " 1 "] {
+            assert!(demo_value_is_truthy(v), "{v:?} should enable demo mode");
+        }
+        // Disabling values.
+        for v in ["", "  ", "0", "false", "False", "no", "off", "OFF"] {
+            assert!(!demo_value_is_truthy(v), "{v:?} should not enable demo mode");
+        }
+    }
+
     #[test]
     fn unknown_double_dash_flag_errors_even_with_positional() {
         // Make sure the path-vs-flag distinction is robust: