feat: add headless browser rendering for JavaScript SPA sites (#321)

New BrowserRenderer class uses Playwright to render JavaScript-heavy
documentation sites (React, Vue SPAs) that return empty HTML shells
with requests.get(). Activated via --browser flag on web scraping.

- browser_renderer.py: Playwright wrapper with lazy browser launch,
  auto-install Chromium on first use, context manager support
- doc_scraper.py: browser_mode config, _render_with_browser() helper,
  integrated into scrape_page() and scrape_page_async()
- SPA detection warnings now suggest --browser flag
- Optional dep: pip install "skill-seekers[browser]"
- 14 real e2e tests (actual Chromium, no mocks)
- UML updated: Scrapers class diagram (BrowserRenderer + dependency),
  Parsers (DoctorParser), Utilities (Doctor), Components, and new
  Browser Rendering sequence diagram (#20)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/UML_ARCHITECTURE.md

@@ -137,7 +137,12 @@ MCP Client (Claude Code/Cursor) → FastMCPServer (stdio/HTTP) with two invocati
 ### Runtime Components
 ![Runtime Components](UML/exports/19_runtime_components.png)
 
-Component diagram with corrected runtime dependencies. Key flows: `CLI Core` dispatches to `Scrapers` (via `scraper.main(argv)`) and to `Adaptors` (via package/upload commands). `Scrapers` call `Codebase Analysis` via `analyze_codebase(enhance_level)`. `Codebase Analysis` uses `C3.x Classes` internally and `Enhancement` when level ≥ 2. `MCP Server` reaches `Scrapers` via subprocess and `Adaptors` via direct import.
+Component diagram with corrected runtime dependencies. Key flows: `CLI Core` dispatches to `Scrapers` (via `scraper.main(argv)`) and to `Adaptors` (via package/upload commands). `Scrapers` call `Codebase Analysis` via `analyze_codebase(enhance_level)`. `Codebase Analysis` uses `C3.x Classes` internally and `Enhancement` when level ≥ 2. `MCP Server` reaches `Scrapers` via subprocess and `Adaptors` via direct import. `Scrapers` optionally use `Browser Renderer (Playwright)` via `render_page()` when `--browser` flag is set for JavaScript SPA sites.
+
+### Browser Rendering Flow
+![Browser Rendering](UML/exports/20_browser_rendering_sequence.png)
+
+When `--browser` flag is set, `DocScraper.scrape_page()` delegates to `BrowserRenderer.render_page(url)` instead of `requests.get()`. The renderer auto-installs Chromium on first use, navigates with `wait_until='networkidle'` to let JavaScript execute, then returns the fully-rendered HTML. The rest of the pipeline (BeautifulSoup → `extract_content()` → `save_page()`) remains unchanged. Optional dependency: `pip install "skill-seekers[browser]"`.
 
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