Tag shell/REPL blocks at build time via asciidoctor extension

Add asciidoctor-extensions/terminal-role.js, an Asciidoctor
tree-processor that walks listing and literal blocks and
calls `block.addRole('terminal')` when the source starts with
a known prompt (iex>, iex(n)>, irb>, irb(...)>, >>, $,
user@host:). The role becomes `class="listingblock terminal"`
in the rendered HTML, which the shared UI bundle styles as a
macOS-style terminal window. Registered via the asciidoc.
extensions entry in antora-playbook.yml and the local
development playbook. Replaces the previous runtime
terminal-detect.js that ran in the browser after DOM load;
now the class is baked into the static HTML at build time.

Author: wintermeyer

antora-local-playbook.yml

@@ -18,3 +18,7 @@ ui:
   output_dir: antora-assets
 runtime:
   fetch: true
+
+asciidoc:
+  extensions:
+    - ./asciidoctor-extensions/terminal-role.js

antora-playbook.yml

@@ -17,3 +17,7 @@ ui:
   output_dir: antora-assets
 runtime:
   fetch: true
+
+asciidoc:
+  extensions:
+    - ./asciidoctor-extensions/terminal-role.js

asciidoctor-extensions/terminal-role.js

@@ -0,0 +1,45 @@
+// Detect shell / REPL transcripts in source and literal blocks
+// at build time and add the `terminal` role to them. The UI
+// bundle's CSS targets `.doc .listingblock.terminal` to render
+// those blocks with macOS-style terminal chrome (three
+// traffic-light dots, subtle depth). Doing this at build time
+// means the class lands in the static HTML — no runtime JS
+// needed in the browser.
+//
+// We match the first non-whitespace line of the block against
+// common prompts:
+//   iex>              — Elixir IEx
+//   iex(n)>           — IEx with numbered prompt
+//   irb>              — Ruby irb (simple form)
+//   irb(main):001:0>  — Ruby irb (default form)
+//   >>                — minimalist prompt used in the Ruby /
+//                       Rails books for irb transcripts
+//   $                 — POSIX shell prompt
+//   user@host:        — shell prompt with hostname prefix
+//
+// The extension is a tree processor: it walks every listing
+// (source) block and every literal block, checks the raw
+// source, and calls `addRole('terminal')` when the prefix
+// matches. Roles on blocks are emitted by Asciidoctor as CSS
+// classes on the wrapping <div>.
+
+'use strict'
+
+const TERMINAL_PREFIX = /^\s*(iex(\([^)]*\))?>|irb(\([^)]*\))?>|>>|\$\s|[A-Za-z0-9_.-]+@[A-Za-z0-9_.-]+[:#$]\s)/
+
+module.exports.register = function register (registry) {
+  registry.treeProcessor(function () {
+    const self = this
+    self.process(function (doc) {
+      ;['listing', 'literal'].forEach(function (context) {
+        doc.findBy({ context: context }).forEach(function (block) {
+          const lines = block.getLines ? block.getLines() : (block.lines || [])
+          const source = Array.isArray(lines) ? lines.join('\n') : String(lines || '')
+          if (source && TERMINAL_PREFIX.test(source)) {
+            block.addRole('terminal')
+          }
+        })
+      })
+    })
+  })
+}