feat: implement three-width layout system (reading, wide, full-bleed)

Replace the narrow 720px column layout with a three-width breakout system:

- Reading width (--width-prose, 720px): prose paragraphs, lists, blockquotes
- Wide width (--width-code, 840px): code blocks, inline SVGs, figures, .wide/.breakout
- Full width (--width-container, 1100px): hero diagrams, .full elements

The outer container is wide; individual content blocks self-constrain via
max-width + margin: auto. This creates the narrow-wide-narrow rhythm.

Changes:
- prose.css: Add .prose > * breakout pattern for three widths, SVG styling
- BaseLayout.astro: Remove max-width from main (children handle their own widths)
- [...slug].astro: Remove --width-prose constraint from .prose-wrapper
- prose.css: Remove old left:50%/translateX(-50%) code block hack

tests/final-verification.test.mjs

@@ -231,10 +231,15 @@ describe('Responsive behavior', () => {
       'Title should use smaller type scale at breakpoints');
   });
 
-  test('code blocks respect gutters on mobile', () => {
+  test('code blocks respect gutters via three-width layout system', () => {
     const prose = readFileSync('src/styles/prose.css', 'utf-8');
-    assert.ok(prose.includes('100vw') && prose.includes('--gutter'),
-      'Code blocks should constrain to viewport minus gutters');
+    const layout = readFileSync('src/layouts/BaseLayout.astro', 'utf-8');
+    // Code blocks self-constrain via max-width in the breakout pattern
+    assert.ok(prose.includes('--width-code'),
+      'Code blocks should use --width-code for wide breakout');
+    // Gutters are provided by main element padding
+    assert.ok(layout.includes('--gutter'),
+      'Main element should provide gutter padding for responsive safety');
   });
 });
 

tests/post-page.test.mjs

@@ -126,10 +126,9 @@ describe('Post page template', () => {
   });
 
   describe('Style', () => {
-    it('should have .prose-wrapper with prose max-width, auto margins, and space-12 bottom padding', () => {
+    it('should have .prose-wrapper with auto margins and space-12 bottom padding (no max-width — children self-constrain)', () => {
       const content = readPage();
       assert.match(content, /\.prose-wrapper/, 'should style .prose-wrapper');
-      assert.match(content, /max-width\s*:\s*var\(--width-prose\)/, 'should have prose max-width');
       assert.match(content, /margin-left\s*:\s*auto/, 'should have auto left margin');
       assert.match(content, /margin-right\s*:\s*auto/, 'should have auto right margin');
       assert.match(content, /padding-bottom\s*:\s*var\(--space-12\)/, 'should have space-12 bottom padding');

tests/three-width-layout.test.mjs

@@ -0,0 +1,119 @@
+import { describe, it } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { readFileSync } from 'node:fs';
+
+const prosePath = 'src/styles/prose.css';
+const layoutPath = 'src/layouts/BaseLayout.astro';
+const postTemplatePath = 'src/pages/posts/[...slug].astro';
+
+function readProse() {
+  return readFileSync(prosePath, 'utf-8');
+}
+
+function readLayout() {
+  return readFileSync(layoutPath, 'utf-8');
+}
+
+function readPostTemplate() {
+  return readFileSync(postTemplatePath, 'utf-8');
+}
+
+describe('Three-width layout system', () => {
+  describe('Reading width (default)', () => {
+    it('should constrain all direct children of .prose to reading width by default', () => {
+      const css = readProse();
+      // .prose > * should have max-width: var(--width-prose)
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*max-width:\s*var\(--width-prose\)/s,
+        '.prose > * should set max-width to --width-prose');
+    });
+
+    it('should center prose children with auto margins', () => {
+      const css = readProse();
+      // .prose > * should have margin-left: auto and margin-right: auto
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*margin-left:\s*auto/s,
+        '.prose > * should have margin-left: auto');
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*margin-right:\s*auto/s,
+        '.prose > * should have margin-right: auto');
+    });
+  });
+
+  describe('Wide width (code blocks, SVGs, figures)', () => {
+    it('should break code blocks to wide width', () => {
+      const css = readProse();
+      // .prose > pre should have max-width: var(--width-code)
+      assert.match(css, /\.prose\s*>\s*pre[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > pre should set max-width to --width-code');
+    });
+
+    it('should break SVGs to wide width', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*svg[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > svg should set max-width to --width-code');
+    });
+
+    it('should break .breakout and .wide elements to wide width', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*\.wide[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > .wide should set max-width to --width-code');
+      assert.match(css, /\.prose\s*>\s*\.breakout[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > .breakout should set max-width to --width-code');
+    });
+  });
+
+  describe('Full width (full-bleed)', () => {
+    it('should support full-bleed elements with .full class', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*\.full[^{]*\{[^}]*max-width:\s*var\(--width-container\)/s,
+        '.prose > .full should set max-width to --width-container');
+    });
+  });
+
+  describe('SVG styling', () => {
+    it('should make SVGs responsive with width 100% and auto height', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*width:\s*100%/s,
+        'SVGs should have width: 100%');
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*height:\s*auto/s,
+        'SVGs should have height: auto');
+    });
+
+    it('should give SVGs proper vertical spacing', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*margin-top:\s*var\(--space-5\)/s,
+        'SVGs should have --space-5 top margin');
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*margin-bottom:\s*var\(--space-5\)/s,
+        'SVGs should have --space-5 bottom margin');
+    });
+
+    it('should give SVGs rounded corners', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*border-radius/s,
+        'SVGs should have border-radius');
+    });
+  });
+
+  describe('BaseLayout main element', () => {
+    it('should not constrain main to --width-container', () => {
+      const content = readLayout();
+      // The main style block should NOT have max-width: var(--width-container)
+      const mainBlock = content.match(/main\s*\{[^}]*\}/s);
+      assert.ok(mainBlock, 'main style block should exist');
+      assert.ok(
+        !mainBlock[0].includes('max-width'),
+        'main should not have max-width — child elements handle their own widths'
+      );
+    });
+  });
+
+  describe('Post template prose-wrapper', () => {
+    it('should not constrain prose-wrapper to --width-prose', () => {
+      const content = readPostTemplate();
+      const styleBlock = content.match(/<style>([\s\S]*?)<\/style>/);
+      assert.ok(styleBlock, 'style block should exist');
+      assert.ok(
+        !styleBlock[1].includes('max-width: var(--width-prose)'),
+        '.prose-wrapper should not be constrained to --width-prose (children self-constrain)'
+      );
+    });
+  });
+});