Add syntax highlighting on code blocks, plus a Code Highlighting post

Colors <pre><code> blocks site-wide via vendored highlight.js v11.11.1
(pinned to that stable tag, not main, which tracks an in-progress
11.0.0-beta1), auto-detected and restricted to
configure({ languages: ['php', 'bash', 'xml', 'css', 'python',
'javascript', 'yaml', 'json', 'ini'] }) - no per-block markup needed for
the ~60 existing code blocks across the site. css/python/javascript ship
in the core bundle; yaml/json/ini (ini covers .env-style files too)
don't and are vendored as separate per-language files.

Themes swap with the existing dark/light toggle: ir-black (dark) +
github (light, resolving the entry's own open question), via the same
data-theme-driven mechanism as the main palette
(syntax-highlight-init.twig/syntax-highlight.twig, mirroring
theme-init.twig/nav.twig's split) - a MutationObserver swaps the theme
link live without touching the existing toggle button's click handler.

Twig-syntax code blocks have no highlight.js grammar and are marked
class="nohighlight" by hand (15 blocks across 9 files, found by grepping
for literal {% %}/{{ }} syntax rather than guessing) rather than
force-matched into the restricted candidate set, which would color them
wrong instead of leaving them plain.

One correction to the original backlog entry's suggested approach: it
suggested vendoring highlight.js under novaconium/vendor/ next to Twig.
That would have silently 404ed on every request - Twig is server-side
PHP, never fetched by a browser, but highlight.js's .js/.css files are,
and only public/ is web-reachable. Vendored to public/vendor/highlightjs/
instead; documented in AGENTS.md and a new upgrading-highlightjs doc,
since public/ isn't touched by the usual novaconium/-swap framework
update workflow, so a future highlight.js bump won't propagate to
existing projects automatically the way it does for everything else
under novaconium/.

Caught two real bugs via testing rather than review: hljs.highlightAll()
silently no-ops if called before the document finishes parsing rather
than deferring itself, and a bash example starting with the word "php"
auto-detects as PHP, not bash.

Also adds App/pages/blog/code-highlighting/ - a new blog post
demonstrating the feature with a verified worked example in each of the
nine languages, plus how to force a language via an explicit
language-<name> class when auto-detection isn't enough.

Closes the "Syntax highlighting on code blocks" backlog item in
novaconium/ISSUES.md.

public/vendor/highlightjs/languages/yaml.min.js

@@ -0,0 +1,28 @@
+/*! `yaml` grammar compiled for Highlight.js 11.11.1 */
+(()=>{var e=(()=>{"use strict";return e=>{
+const n="true false yes no null",a="[\\w#;/?:@&=+$,.~*'()[\\]]+",s={
+className:"string",relevance:0,variants:[{begin:/"/,end:/"/},{begin:/\S+/}],
+contains:[e.BACKSLASH_ESCAPE,{className:"template-variable",variants:[{
+begin:/\{\{/,end:/\}\}/},{begin:/%\{/,end:/\}/}]}]},i=e.inherit(s,{variants:[{
+begin:/'/,end:/'/,contains:[{begin:/''/,relevance:0}]},{begin:/"/,end:/"/},{
+begin:/[^\s,{}[\]]+/}]}),l={end:",",endsWithParent:!0,excludeEnd:!0,keywords:n,
+relevance:0},t={begin:/\{/,end:/\}/,contains:[l],illegal:"\\n",relevance:0},c={
+begin:"\\[",end:"\\]",contains:[l],illegal:"\\n",relevance:0},r=[{
+className:"attr",variants:[{begin:/[\w*@][\w*@ :()\./-]*:(?=[ \t]|$)/},{
+begin:/"[\w*@][\w*@ :()\./-]*":(?=[ \t]|$)/},{
+begin:/'[\w*@][\w*@ :()\./-]*':(?=[ \t]|$)/}]},{className:"meta",
+begin:"^---\\s*$",relevance:10},{className:"string",
+begin:"[\\|>]([1-9]?[+-])?[ ]*\\n( +)[^ ][^\\n]*\\n(\\2[^\\n]+\\n?)*"},{
+begin:"<%[%=-]?",end:"[%-]?%>",subLanguage:"ruby",excludeBegin:!0,excludeEnd:!0,
+relevance:0},{className:"type",begin:"!\\w+!"+a},{className:"type",
+begin:"!<"+a+">"},{className:"type",begin:"!"+a},{className:"type",begin:"!!"+a
+},{className:"meta",begin:"&"+e.UNDERSCORE_IDENT_RE+"$"},{className:"meta",
+begin:"\\*"+e.UNDERSCORE_IDENT_RE+"$"},{className:"bullet",begin:"-(?=[ ]|$)",
+relevance:0},e.HASH_COMMENT_MODE,{beginKeywords:n,keywords:{literal:n}},{
+className:"number",
+begin:"\\b[0-9]{4}(-[0-9][0-9]){0,2}([Tt \\t][0-9][0-9]?(:[0-9][0-9]){2})?(\\.[0-9]*)?([ \\t])*(Z|[-+][0-9][0-9]?(:[0-9][0-9])?)?\\b"
+},{className:"number",begin:e.C_NUMBER_RE+"\\b",relevance:0},t,c,{
+className:"string",relevance:0,begin:/'/,end:/'/,contains:[{match:/''/,
+scope:"char.escape",relevance:0}]},s],g=[...r]
+;return g.pop(),g.push(i),l.contains=g,{name:"YAML",case_insensitive:!0,
+aliases:["yml"],contains:r}}})();hljs.registerLanguage("yaml",e)})();