feat: PDF export via headless Chrome with Playwright fallback

[YashvantHange]

Adds investo analyze --pdf, rendering the RFC 2 research note to PDF. Third of four RFCs; depends on #14 (merged).

No new required dependency

PDF is genuinely optional at the dependency level. investo.export.html_to_pdf tries, in order:

1. A system Chrome / Edge / Chromium / Brave (discovered by platform path or PATH, or INVESTO_CHROME) — the common case, zero setup.
2. Playwright-managed Chromium, if pip install 'investo[pdf]' and playwright install chromium have been run.
3. Otherwise a PdfExportError naming all three remedies — while the .html is still written, so a failure leaves a usable report, not an empty hand.

Four headless-Chrome details that were each a real bug

Commented at their site in _chrome_pdf, because every one is a silent failure:

• TemporaryDirectory + a file inside it, not NamedTemporaryFile — on Windows a still-open NamedTemporaryFile can't be reopened by path, which is exactly what Chrome must do.
• resolve().as_uri() for the URL — yields file:///C:/… with spaces percent-escaped; "file://" + str(path) gets both wrong.
• A mandatory throwaway --user-data-dir — without it, headless can attach to an already-running browser and silently produce nothing (the docs: add Cursor Directory listing (badge + one-click install) #1 "works in CI, not locally" cause).
• Verify the file exists and is non-empty — do not trust the exit code. --print-to-pdf can exit 0 having written nothing. There's a dedicated test for this case.

CLI flags now compose

--json, --html, --pdf each do one thing and combine — previously --html silently suppressed --json. Bare --html/--pdf write investo-<SYMBOL>-<date>.<ext> (ticker sanitised — M&M.NS is real), parents are created, and a PDF failure exits 2 with the sidecar kept and the reason on stderr.

Verification

ruff + mypy clean; 223 tests pass (was 198). The new test_export.py and test_cli.py never launch a real browser — every test stubs subprocess.run or Playwright.

The real proof, though, is a live run: investo analyze "KPIT Technologies" --pdf produced a valid 7-page A4 PDF (%PDF-1.4, MediaBox 595×842pt, embedded fonts, clean %%EOF) through headless Chrome on this machine, with the .html sidecar alongside.

⚠️ Reviewer note: Chrome's --print-to-pdf has no @page margin-box support, so there are no CSS page numbers — running header/footer only (via position: fixed). That's a documented engine limit, not an omission.

No breaking changes

--pdf is new; the pdf extra is optional; new config fields have defaults.