feat: adopt strict TDD framework and add adversarial test suite

- Update AGENTS.md and .agents/rules/blog-system.md with strict TDD mandate
- Extract Eleventy custom filters into modular src/_filters/index.js for unit testing
- Add npm test script using Node.js native test runner (node:test)
- Create test/site-config.test.js for environment validation and fatal error assertions
- Create test/filters.test.js for adversarial filter edge cases (dates, limits, reading time, headings)
- Create test/frontmatter-schema.test.js enforcing post metadata contracts
- Create test/design-rules.test.js enforcing strict anti-patterns (no glassmorphism, no shadows, max 4px radius)
- Create test/build-integrity.test.js checking static output, zero 404 links, and TOC anchor targets

test/design-rules.test.js

@@ -0,0 +1,86 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+
+const cssDir = path.resolve("src/assets/css");
+
+function getAllCssFiles(dir) {
+  let results = [];
+  const list = fs.readdirSync(dir);
+  for (const file of list) {
+    const filePath = path.join(dir, file);
+    const stat = fs.statSync(filePath);
+    if (stat && stat.isDirectory()) {
+      results = results.concat(getAllCssFiles(filePath));
+    } else if (file.endsWith(".css")) {
+      results.push(filePath);
+    }
+  }
+  return results;
+}
+
+test("Design Specification: Strict Anti-Pattern Regression Testing", () => {
+  const cssFiles = getAllCssFiles(cssDir);
+  assert.ok(cssFiles.length >= 5, "Must inspect all modular CSS files");
+
+  for (const file of cssFiles) {
+    const relPath = path.relative(process.cwd(), file);
+    const content = fs.readFileSync(file, "utf-8");
+
+    // 1. Anti-Pattern: NO Glassmorphism (backdrop-filter)
+    assert.doesNotMatch(
+      content,
+      /backdrop-filter\s*:/i,
+      `Violation in ${relPath}: Glassmorphism (backdrop-filter) is strictly forbidden`
+    );
+
+    // 2. Anti-Pattern: NO Drop Shadows or Glows (box-shadow, text-shadow)
+    // Allowed: box-shadow: none; or text-shadow: none;
+    const shadowMatches = content.match(/(box-shadow|text-shadow)\s*:\s*([^;]+);/gi) || [];
+    for (const match of shadowMatches) {
+      assert.match(
+        match,
+        /(box-shadow|text-shadow)\s*:\s*none\s*;/i,
+        `Violation in ${relPath}: Drop shadows and glows are forbidden (found: '${match}')`
+      );
+    }
+
+    // 3. Anti-Pattern: NO Gradients (linear, radial, conic)
+    assert.doesNotMatch(
+      content,
+      /(linear-gradient|radial-gradient|conic-gradient)/i,
+      `Violation in ${relPath}: Gradients are strictly forbidden`
+    );
+
+    // 4. Anti-Pattern: NO Rounded Bubbles (border-radius capped at 4px max)
+    const radiusMatches = content.match(/border-radius\s*:\s*([^;]+);/gi) || [];
+    for (const match of radiusMatches) {
+      // Check for pixel values > 4px (e.g. 5px, 6px, 8px, 12px, 20px)
+      const pxMatch = match.match(/(\d+)\s*px/i);
+      if (pxMatch) {
+        const px = parseInt(pxMatch[1], 10);
+        assert.ok(
+          px <= 4,
+          `Violation in ${relPath}: Border radius must be 0px to 4px max, found: ${px}px in '${match}'`
+        );
+      }
+      // Check for percentages (e.g. 50%)
+      assert.doesNotMatch(
+        match,
+        /\d+%/,
+        `Violation in ${relPath}: Rounded pill/circular percentages forbidden in '${match}'`
+      );
+    }
+
+    // 5. Anti-Pattern: NO Float/Spring Transitions (Must be instant snaps)
+    const transitionMatches = content.match(/transition\s*:\s*([^;]+);/gi) || [];
+    for (const match of transitionMatches) {
+      assert.match(
+        match,
+        /transition\s*:\s*(none(\s*!important)?|0s\s*);/i,
+        `Violation in ${relPath}: Transitions must be instant (none or 0s), found: '${match}'`
+      );
+    }
+  }
+});