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/app.rs

@@ -368,6 +368,14 @@ pub struct App {
     /// flag; the `undo` / `redo` commands then report undo is off
     /// rather than emitting a prepare action.
     pub undo_enabled: bool,
+    /// Whether **demonstration mode** is active this session (ADR-0047,
+    /// issue #22). `true` under `--demo` / `RDBMS_PLAYGROUND_DEMO`. When
+    /// off (the default) none of the demo key handling or overlay
+    /// rendering runs — zero footprint. When on, otherwise-invisible
+    /// keys raise a transient badge (set by the runtime, see
+    /// `demo_badge`) and `Ctrl+]` drives the stealth step-caption
+    /// buffer (`demo_caption` / `demo_capturing`).
+    pub demo_mode: bool,
     /// The DSL → SQL teaching echo (ADR-0038) for the command currently
     /// being rendered: set from the success event just before its handler
     /// runs, consumed by `note_ok_summary` (which pushes it beneath
@@ -512,6 +520,9 @@ impl App {
             // Undo is on by default; the runtime flips this off for
             // a `--no-undo` session (ADR-0006 Amendment 1).
             undo_enabled: true,
+            // Demo mode is off by default; the runtime flips it on for
+            // a `--demo` session (ADR-0047).
+            demo_mode: false,
             pending_echo: None,
         }
     }

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:

src/friendly/strings/en-US.yaml

@@ -204,6 +204,9 @@ help:
                                  project's stored mode. Without it, the
                                  project's last-used mode is restored
                                  (default: simple).
+            --demo               Demonstration mode: show on-screen badges
+                                 for otherwise-invisible keys (Tab, Enter,
+                                 ...) — for screencasts and live teaching.
 
     App-level commands (typed inside the app, available in both modes):
         quit                     Exit cleanly.

src/runtime.rs

@@ -216,6 +216,9 @@ pub async fn run(args: Args) -> Result<()> {
     let db_existed = db_path.exists();
     // Undo is on unless `--no-undo` (ADR-0006 Amendment 1).
     let undo_enabled = !args.no_undo;
+    // Demonstration mode under `--demo` / `RDBMS_PLAYGROUND_DEMO`
+    // (ADR-0047). Off by default; threaded onto the `App` in run_loop.
+    let demo_mode = args.demo;
     let database =
         Database::open_with_persistence_and_undo(db_path.as_path(), persistence, undo_enabled)
             .context("open database")?;
@@ -273,6 +276,7 @@ pub async fn run(args: Args) -> Result<()> {
         initial_events,
         undo_enabled,
         resolved_mode,
+        demo_mode,
     )
     .await;
     if let Err(e) = teardown_terminal(&mut terminal) {
@@ -331,6 +335,7 @@ async fn run_loop(
     initial_events: Vec<AppEvent>,
     undo_enabled: bool,
     initial_mode: crate::mode::Mode,
+    demo_mode: bool,
 ) -> Result<Option<String>> {
     let (event_tx, mut event_rx) = mpsc::channel::<AppEvent>(EVENT_CHANNEL_CAPACITY);
     let reader_handle = spawn_event_reader(event_tx.clone());
@@ -339,6 +344,8 @@ async fn run_loop(
     app.project_name = Some(project_display_name);
     app.project_is_temp = project_is_temp;
     app.undo_enabled = undo_enabled;
+    // ADR-0047: enable the demo overlays for this session under `--demo`.
+    app.demo_mode = demo_mode;
     // Start in the resolved input mode (ADR-0015 mode-restore
     // amendment, issue #14): `--mode` > stored project mode >
     // default. `Persistence` already carries the same value, so the