[Tearsheet] PNG export — 1200x900 image for README embed and social sharing

[DogInfantry]

Part of epic: #30
Depends on: #33 (HTML render must exist first)

What

Add render_png() method to TearsheetGenerator that takes a headless screenshot of the HTML tearsheet and saves it as a 1200x900px PNG. This image is embedded directly in the README as the primary demo artifact — a recruiter sees the output quality before running a single line of code.

Implementation options (choose one)

Option A — playwright (preferred)

def render_png(self, output_path: str) -> None:
    from playwright.sync_api import sync_playwright
    html = self.render_html()
    with sync_playwright() as p:
        browser = p.chromium.launch()
        page = browser.new_page(viewport={'width': 1200, 'height': 900})
        page.set_content(html)
        page.screenshot(path=output_path, full_page=False)
        browser.close()

Option B — selenium (fallback if playwright not available)

Acceptance Criteria

• render_png(path) saves a 1200x900px PNG without manual browser interaction
• PNG is committed to /outputs/demo/tearsheet_AAPL.png
• README ![Tearsheet Demo](outputs/demo/tearsheet_AAPL.png) embed works on GitHub
• playwright or selenium added to requirements-dev.txt (not main, since it's optional)
• Graceful fallback if headless browser not available (warn, skip, don't crash pipeline)