feat: add browsable peptide directory with category filtering, REST API, and extended meta fields (v4.3.0)

Adds [peptide_directory] shortcode with card grid, category taxonomy (8 terms),
search, pagination, detail modal, and /v1/compounds REST endpoint. Includes 9
new meta fields (half-life, stability, solubility, etc.), admin data tools for
migration/re-enrichment, integration hooks for cross-plugin extensibility,
dark mode support, and full uninstall cleanup. Refactors AI generator into
PSA_OpenRouter + PSA_KB_Builder classes. Adds PHPUnit tests for PostType and
Directory classes. Fixes QA findings: dead JS selector, renderError loading
state, numeric meta validation, prepared SQL in uninstall.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -49,6 +49,28 @@ jobs:
         run: |
           composer run lint
 
+  # ── PHPUnit Tests ────────────────────────────────────────────────
+  test-phpunit:
+    name: PHPUnit Tests (PHP ${{ matrix.php }})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php: ['7.4', '8.1', '8.3']
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up PHP ${{ matrix.php }}
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          tools: composer
+
+      - name: Install Composer dependencies
+        run: composer install --no-interaction --prefer-dist --no-progress
+
+      - name: Run PHPUnit tests
+        run: composer run test
+
   # ── JavaScript Lint ──────────────────────────────────────────────
   lint-js:
     name: JavaScript Linting