docs(website): document m:n, --demo, schema sidebar, responsive input

Document the features the main merge shipped: `create m:n relationship`
(relationships ref + build-the-library note), the `--demo` teaching
flag (command-line-options), the Ctrl-O schema sidebar (output-pane,
now .mdx to embed the new cast), and horizontal/two-row input
(assistive-editor).

website/src/content/docs/using-the-playground/command-line-options.md

@@ -27,6 +27,7 @@ rdbms-playground path/to/project
 | `--mode <simple\|advanced>` | Start in this input mode, overriding the project's stored mode (precedence: `--mode` > stored > simple). |
 | `--theme <light\|dark>` | Force a theme instead of auto-detecting from the terminal. |
 | `--no-undo` | Disable the undo machinery for this run — no snapshot is taken before each change (see [Undo, redo & history](/using-the-playground/undo-and-history/)). |
+| `--demo` | Turn on demonstration mode — a teaching aid that briefly shows an on-screen badge for keys that otherwise leave no visible mark (Tab, Enter, the arrows, and the like). Useful for screencasts and for demonstrating the playground to a class. Off by default. |
 | `--log-file <PATH>` | Write diagnostic logging to `PATH`. |
 | `-h`, `--help` | Print the usage banner and exit. |
 

website/src/content/docs/using-the-playground/the-assistive-editor.mdx

@@ -52,6 +52,10 @@ by character, <kbd>Home</kbd>/<kbd>End</kbd> to the ends, and
 <kbd>Delete</kbd>/<kbd>Backspace</kbd> to remove characters. Your
 project-scoped command history is available with <kbd>↑</kbd>/<kbd>↓</kbd>.
 
+A long command does not get cut off: the line scrolls sideways to keep the
+cursor in view, and on a tall terminal the input field uses two rows so you
+can see more of it at once.
+
 :::note[Planned]
 Multi-line entry and extra readline-style shortcuts (Ctrl-A/E/W/K/U) are
 planned and not yet available.

website/src/content/docs/using-the-playground/the-output-pane.md

@@ -1,22 +0,0 @@
----
-title: The output pane
-description: Where results appear, and how to scroll back through them.
-sidebar:
-  order: 3
----
-
-Results appear in the output pane: the structure of a table after you change
-it, the rows from `show data` and after writes (as aligned, box-drawn
-tables), query plans, and command outcomes marked with a ✓ or ✗.
-
-## Scrolling
-
-Use <kbd>PageUp</kbd> and <kbd>PageDown</kbd> to scroll back through earlier
-output. New output snaps the view back to the most recent entry, so you never
-lose your place when you run something.
-
-:::note[Planned]
-A fuller session journal — a scrollable, richly rendered log of the whole
-session that you can save as Markdown — and multiple result tabs are planned
-and not yet available.
-:::

website/src/content/docs/using-the-playground/the-output-pane.mdx

@@ -0,0 +1,41 @@
+---
+title: The output pane
+description: Where results appear, the schema sidebar, and how to scroll back through them.
+sidebar:
+  order: 3
+---
+
+import Demo from '../../../components/Demo.astro';
+
+Results appear in the output pane: the structure of a table after you change
+it, the rows from `show data` and after writes (as aligned, box-drawn
+tables), query plans, and command outcomes marked with a ✓ or ✗.
+
+## Scrolling
+
+Use <kbd>PageUp</kbd> and <kbd>PageDown</kbd> to scroll back through earlier
+output. New output snaps the view back to the most recent entry, so you never
+lose your place when you run something.
+
+## The schema sidebar
+
+On a wide terminal a sidebar runs down the left-hand side, listing your
+**tables** and **relationships** so the shape of your database is always in
+view. It shows and hides itself automatically with the terminal width — on a
+narrow window it stays out of the way and the output pane gets the full width.
+
+Press <kbd>Ctrl-O</kbd> to bring the sidebar up at any size and step through
+it: the first press focuses the **Tables** panel, the next focuses
+**Relationships**, and once more returns you to the input field
+(<kbd>Esc</kbd> leaves it directly). While a panel is focused it widens to
+show more detail and is marked with a coloured border;
+<kbd>↑</kbd>/<kbd>↓</kbd> scroll it a line at a time and
+<kbd>PageUp</kbd>/<kbd>PageDown</kbd> a page at a time.
+
+<Demo src="/casts/schema-sidebar.cast" title="Ctrl-O brings up the schema sidebar and steps through the Tables and Relationships panels." />
+
+:::note[Planned]
+A fuller session journal — a scrollable, richly rendered log of the whole
+session that you can save as Markdown — and multiple result tabs are planned
+and not yet available.
+:::