Add client-side shell syntax highlighter for Aliki theme

Add a simple shell/bash syntax highlighter for documentation code blocks.
The highlighter is focused on command-line documentation rather than full
bash scripts.

Highlights:
- $ prompts (gray)
- Commands/executables - first word on line (blue)
- Options like -f, --flag, --option=value (cyan)
- Single and double quoted strings (green)
- Comments starting with # (gray, italic)

The highlighter targets code blocks with language classes: bash, sh, shell,
console.

Files added:
- lib/rdoc/generator/template/aliki/js/bash_highlighter.js
- test/rdoc/generator/aliki/highlight_bash_test.rb

Author: st0012

lib/rdoc/generator/template/aliki/_head.rhtml

@@ -140,6 +140,11 @@
   defer
 ></script>
 
+<script
+  src="<%= h asset_rel_prefix %>/js/bash_highlighter.js?v=<%= h RDoc::VERSION %>"
+  defer
+></script>
+
 <script
   src="<%= h asset_rel_prefix %>/js/aliki.js?v=<%= h RDoc::VERSION %>"
   defer

lib/rdoc/generator/template/aliki/css/rdoc.css

@@ -1059,6 +1059,18 @@ main h6 a:hover {
   font-style: italic;
 }
 
+/* Shell Syntax Highlighting */
+.sh-prompt { color: var(--code-gray); }
+.sh-command { color: var(--code-blue); }
+.sh-option { color: var(--code-cyan); }
+.sh-string { color: var(--code-green); }
+.sh-envvar { color: var(--code-purple); }
+
+.sh-comment {
+  color: var(--code-gray);
+  font-style: italic;
+}
+
 /* Emphasis */
 em {
   text-decoration-color: var(--color-emphasis-decoration);

lib/rdoc/generator/template/aliki/js/bash_highlighter.js

@@ -0,0 +1,167 @@
+/**
+ * Client-side shell syntax highlighter for RDoc
+ * Highlights: $ prompts, commands, options, strings, env vars, comments
+ */
+
+(function() {
+  'use strict';
+
+  function escapeHtml(text) {
+    return text
+      .replace(/&/g, '&')
+      .replace(/</g, '&lt;')
+      .replace(/>/g, '&gt;')
+      .replace(/"/g, '&quot;')
+      .replace(/'/g, '&#39;');
+  }
+
+  function wrap(className, text) {
+    return '<span class="' + className + '">' + escapeHtml(text) + '</span>';
+  }
+
+  function highlightLine(line) {
+    if (line.trim() === '') return escapeHtml(line);
+
+    var result = '';
+    var i = 0;
+    var len = line.length;
+
+    // Preserve leading whitespace
+    while (i < len && (line[i] === ' ' || line[i] === '\t')) {
+      result += escapeHtml(line[i++]);
+    }
+
+    // Check for $ prompt ($ followed by space or end of line)
+    if (line[i] === '$' && (line[i + 1] === ' ' || line[i + 1] === undefined)) {
+      result += wrap('sh-prompt', '$');
+      i++;
+    }
+
+    // Check for # comment at start
+    if (line[i] === '#') {
+      return result + wrap('sh-comment', line.slice(i));
+    }
+
+    var seenCommand = false;
+    var afterSpace = true;
+
+    while (i < len) {
+      var ch = line[i];
+
+      // Whitespace
+      if (ch === ' ' || ch === '\t') {
+        result += escapeHtml(ch);
+        i++;
+        afterSpace = true;
+        continue;
+      }
+
+      // Comment after whitespace
+      if (ch === '#' && afterSpace) {
+        result += wrap('sh-comment', line.slice(i));
+        break;
+      }
+
+      // Double-quoted string
+      if (ch === '"') {
+        var end = i + 1;
+        while (end < len && line[end] !== '"') {
+          if (line[end] === '\\' && end + 1 < len) end += 2;
+          else end++;
+        }
+        if (end < len) end++;
+        result += wrap('sh-string', line.slice(i, end));
+        i = end;
+        afterSpace = false;
+        continue;
+      }
+
+      // Single-quoted string
+      if (ch === "'") {
+        var end = i + 1;
+        while (end < len && line[end] !== "'") end++;
+        if (end < len) end++;
+        result += wrap('sh-string', line.slice(i, end));
+        i = end;
+        afterSpace = false;
+        continue;
+      }
+
+      // Environment variable (ALLCAPS=)
+      if (afterSpace && /[A-Z]/.test(ch)) {
+        var match = line.slice(i).match(/^[A-Z][A-Z0-9_]*=/);
+        if (match) {
+          result += wrap('sh-envvar', match[0]);
+          i += match[0].length;
+          // Read unquoted value
+          var valEnd = i;
+          while (valEnd < len && line[valEnd] !== ' ' && line[valEnd] !== '\t' && line[valEnd] !== '"' && line[valEnd] !== "'") valEnd++;
+          if (valEnd > i) {
+            result += escapeHtml(line.slice(i, valEnd));
+            i = valEnd;
+          }
+          afterSpace = false;
+          continue;
+        }
+      }
+
+      // Option (must be after whitespace)
+      if (ch === '-' && afterSpace) {
+        var match = line.slice(i).match(/^--?[a-zA-Z0-9_-]+(=[^"'\s]*)?/);
+        if (match) {
+          result += wrap('sh-option', match[0]);
+          i += match[0].length;
+          afterSpace = false;
+          continue;
+        }
+      }
+
+      // Command (first word: regular, ./path, ../path, ~/path, /abs/path, @scope/pkg)
+      if (!seenCommand && afterSpace) {
+        var isCmd = /[a-zA-Z0-9@~\/]/.test(ch) ||
+                    (ch === '.' && (line[i + 1] === '/' || (line[i + 1] === '.' && line[i + 2] === '/')));
+        if (isCmd) {
+          var end = i;
+          while (end < len && line[end] !== ' ' && line[end] !== '\t') end++;
+          result += wrap('sh-command', line.slice(i, end));
+          i = end;
+          seenCommand = true;
+          afterSpace = false;
+          continue;
+        }
+      }
+
+      // Everything else
+      result += escapeHtml(ch);
+      i++;
+      afterSpace = false;
+    }
+
+    return result;
+  }
+
+  function highlightShell(code) {
+    return code.split('\n').map(highlightLine).join('\n');
+  }
+
+  function initHighlighting() {
+    var selectors = [
+      'pre.bash', 'pre.sh', 'pre.shell', 'pre.console',
+      'pre[data-language="bash"]', 'pre[data-language="sh"]',
+      'pre[data-language="shell"]', 'pre[data-language="console"]'
+    ];
+
+    var blocks = document.querySelectorAll(selectors.join(', '));
+    blocks.forEach(function(block) {
+      if (block.getAttribute('data-highlighted') === 'true') return;
+      block.innerHTML = highlightShell(block.textContent);
+      block.setAttribute('data-highlighted', 'true');
+    });
+  }
+
+  if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', initHighlighting);
+  } else {
+    initHighlighting();
+  }
+})();