feat: simple-mode code-block highlighting, prompt, and copy rules

Add a custom Shiki grammar for the simple-mode command language
(src/grammars/rdbms.mjs), registered with Expressive Code. Two language ids
share it: rdbms (real commands) and rdbms-syntax (abstract templates).
Simple-mode blocks now highlight; advanced examples keep sql.

Separation + copy ergonomics via CSS (global.css): a decorative, copy-safe
"> " prompt on rdbms command lines (not in the copy buffer), and the copy
button hidden on multi-command rdbms blocks and on rdbms-syntax templates
(the app input is single-line, so a multi-command paste is not runnable);
single-command, sql, and sh blocks keep copy.

Content: convert 22 simple-mode fences to rdbms; lead the simplest examples
(first project, Tables reference) with bare "with pk" (the beginner default
that creates a ready-made id key), pointing to the named form. Record the
fence + prompt conventions in STYLE.md.

website/src/content/docs/guides/build-the-library.md

@@ -15,9 +15,13 @@ from scratch in simple mode: two tables, a relationship between them, and a
 few rows. By the end you will have used the create → add column → relate →
 insert → query loop end to end.
 
+We give each table a **named** primary key (like `author_id`) so the
+relationship reads clearly — but `with pk` on its own gives you a default
+`id` if you prefer, as in [Your first project](/getting-started/first-project/).
+
 ## 1. Create the authors table
 
-```text
+```rdbms
 create table authors with pk author_id(serial)
 add column to authors: name (text)
 add column to authors: birth_year (int)
@@ -25,7 +29,7 @@ add column to authors: birth_year (int)
 
 ## 2. Create the books table
 
-```text
+```rdbms
 create table books with pk book_id(serial)
 add column to books: title (text)
 add column to books: author_id (int)
@@ -37,7 +41,7 @@ add column to books: published (int)
 An author has many books, so `books.author_id` should point at
 `authors.author_id`. Declare that one-to-many relationship:
 
-```text
+```rdbms
 add 1:n relationship from authors.author_id to books.author_id
 ```
 
@@ -48,7 +52,7 @@ the `books` side.
 
 `author_id` and `book_id` are `serial`, so they fill themselves in:
 
-```text
+```rdbms
 insert into authors (name, birth_year) values ('Ada Lovelace', 1815)
 insert into authors (name, birth_year) values ('Alan Turing', 1912)
 insert into books (title, author_id, published) values ('Notes on the Analytical Engine', 1, 1843)
@@ -56,7 +60,7 @@ insert into books (title, author_id, published) values ('Notes on the Analytical
 
 ## 5. Look at your data
 
-```text
+```rdbms
 show data authors
 show data books
 ```