feat: integrate syntax highlighting into render pipeline

- src/render.rs: Rewrite with event-based pulldown-cmark parsing.
  Intercepts CodeBlock events and applies tree-sitter highlighting.
  4 new tests covering highlighted and unhighlighted code blocks.

- src/highlight.rs: Fix HtmlRenderer callback to return attributes
  only (not full span tags). Static HTML_ATTRS array for zero-alloc.

- static/style.css: Add 20 hl-* CSS classes with CSS variables.
  Light and dark mode via prefers-color-scheme.

Supported languages: rust, bash, json. Phase 2 complete.

static/style.css

@@ -40,7 +40,9 @@
 }
 
 /* Reset */
-*, *::before, *::after {
+*,
+*::before,
+*::after {
   box-sizing: border-box;
   margin: 0;
   padding: 0;
@@ -62,7 +64,9 @@ body {
 }
 
 /* Layout */
-nav, main, footer {
+nav,
+main,
+footer {
   width: 100%;
   max-width: var(--max-width);
   margin: 0 auto;
@@ -89,16 +93,28 @@ footer {
 }
 
 /* Typography */
-h1, h2, h3 {
+h1,
+h2,
+h3 {
   line-height: 1.3;
   margin-block: var(--space-lg) var(--space-md);
 }
 
-h1 { font-size: 2rem; }
-h2 { font-size: 1.5rem; }
-h3 { font-size: 1.25rem; }
+h1 {
+  font-size: 2rem;
+}
 
-h1:first-child, h2:first-child, h3:first-child {
+h2 {
+  font-size: 1.5rem;
+}
+
+h3 {
+  font-size: 1.25rem;
+}
+
+h1:first-child,
+h2:first-child,
+h3:first-child {
   margin-top: 0;
 }
 
@@ -117,7 +133,8 @@ a:hover {
 }
 
 /* Code */
-code, pre {
+code,
+pre {
   font-family: var(--font-mono);
   font-size: 0.9em;
 }
@@ -142,7 +159,8 @@ pre code {
 }
 
 /* Lists */
-ul, ol {
+ul,
+ol {
   margin-block: var(--space-md);
   padding-left: var(--space-lg);
 }
@@ -249,3 +267,117 @@ article.post header {
   font-size: 1.25rem;
   color: var(--text-muted);
 }
+
+/* Syntax Highlighting */
+:root {
+  --hl-keyword: #d73a49;
+  --hl-string: #22863a;
+  --hl-comment: #6a737d;
+  --hl-function: #6f42c1;
+  --hl-type: #005cc5;
+  --hl-number: #005cc5;
+  --hl-operator: #d73a49;
+  --hl-variable: #24292e;
+  --hl-constant: #005cc5;
+  --hl-property: #005cc5;
+  --hl-punctuation: #24292e;
+  --hl-attribute: #22863a;
+}
+
+@media (prefers-color-scheme: dark) {
+  :root {
+    --hl-keyword: #f97583;
+    --hl-string: #9ecbff;
+    --hl-comment: #6a737d;
+    --hl-function: #b392f0;
+    --hl-type: #79b8ff;
+    --hl-number: #79b8ff;
+    --hl-operator: #f97583;
+    --hl-variable: #e1e4e8;
+    --hl-constant: #79b8ff;
+    --hl-property: #79b8ff;
+    --hl-punctuation: #e1e4e8;
+    --hl-attribute: #85e89d;
+  }
+}
+
+.hl-keyword {
+  color: var(--hl-keyword);
+}
+
+.hl-string {
+  color: var(--hl-string);
+}
+
+.hl-comment {
+  color: var(--hl-comment);
+  font-style: italic;
+}
+
+.hl-function {
+  color: var(--hl-function);
+}
+
+.hl-function-builtin {
+  color: var(--hl-function);
+}
+
+.hl-type {
+  color: var(--hl-type);
+}
+
+.hl-type-builtin {
+  color: var(--hl-type);
+}
+
+.hl-number {
+  color: var(--hl-number);
+}
+
+.hl-operator {
+  color: var(--hl-operator);
+}
+
+.hl-variable {
+  color: var(--hl-variable);
+}
+
+.hl-variable-builtin {
+  color: var(--hl-variable);
+}
+
+.hl-variable-parameter {
+  color: var(--hl-variable);
+}
+
+.hl-constant {
+  color: var(--hl-constant);
+}
+
+.hl-constant-builtin {
+  color: var(--hl-constant);
+}
+
+.hl-property {
+  color: var(--hl-property);
+}
+
+.hl-punctuation {
+  color: var(--hl-punctuation);
+}
+
+.hl-punctuation-bracket {
+  color: var(--hl-punctuation);
+}
+
+.hl-punctuation-delimiter {
+  color: var(--hl-punctuation);
+}
+
+.hl-attribute {
+  color: var(--hl-attribute);
+}
+
+.hl-constructor {
+  color: var(--hl-type);
+}