Iteration 264: Add Styler (DataFrame.style API)

Port pandas.DataFrame.style / pandas.io.formats.style.Styler to TypeScript.

New module src/stats/style.ts provides:
- Styler class with fluent chaining API
- dataFrameStyle(df) factory function
- format / formatIndex / setPrecision / setNaRep
- apply (axis-wise) / applymap / map (element-wise)
- highlightMax / highlightMin / highlightNull / highlightBetween
- backgroundGradient / textGradient / barChart
- setCaption / setTableStyles / setTableAttributes / setProperties
- hide (index or columns)
- toHtml / render / toLatex
- exportStyles / clearStyles

Tests: 50+ test cases including unit, property-based (fast-check).
Playground: playground/style.html with complete API reference.

Run: https://github.com/githubnext/tsessebe/actions/runs/24838264967

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

playground/index.html

@@ -449,6 +449,11 @@ <h3><a href="testing.html" style="color: var(--accent); text-decoration: none;">
           <p>assertSeriesEqual / assertFrameEqual / assertIndexEqual — rich assertion helpers for use in test suites. Numeric tolerance, checkLike column-order mode, dtype checks, AssertionError with detailed diff messages. Mirrors pandas.testing.</p>
           <div class="status done">✅ Complete</div>
         </div>
+        <div class="feature-card">
+          <h3><a href="style.html" style="color: var(--accent); text-decoration: none;">🎨 Styler — DataFrame Style API</a></h3>
+          <p>dataFrameStyle(df) · highlightMax / highlightMin / highlightNull / highlightBetween · backgroundGradient / textGradient · barChart · format / formatIndex · apply / applymap / map · setCaption / setTableStyles / hide · toHtml / toLatex. Mirrors pandas.DataFrame.style (Styler).</p>
+          <div class="status done">✅ Complete</div>
+        </div>
     </section>
       <div class="features-grid">
         <div class="feature-card">