docs: add API references, mapping corrections, and verification script

- Add yfinance.org and defeatbeta-api.org reference docs
- Fix defeatbeta_mapping.org: deprecated yfinance property names
  (quarterly_financials→quarterly_income_stmt, financials→income_stmt),
  longName vs longBusinessSummary conceptual mismatch, cashflow note typo
- Add Mapping Limitations section with live verification results (AAPL):
  DuckDB 1.4.3 incompatibility, format differences, coverage gaps
- Add docs/test_mapping.py as runnable mapping verification script
- Add offline.py, persistent_cache.py, download_data.py, warmup_cache.py
  for offline/cached defeatbeta usage
- Add aapl_yfinance.py exploration script and quant.py scaffold
- Add .envrc (uv layout) and update pyproject.toml + uv.lock

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

download_data.py

@@ -0,0 +1,75 @@
+"""
+One-time download of all defeatbeta parquet files + company_tickers.json.
+Run this once; after that use offline.py for zero-network Ticker() calls.
+
+    uv run python download_data.py
+    uv run python download_data.py --out data/parquet  # custom directory
+"""
+import argparse
+import sys
+from pathlib import Path
+
+import requests
+
+BASE = "https://huggingface.co/datasets/defeatbeta/yahoo-finance-data/resolve/main"
+
+PARQUET_TABLES = [
+    "stock_profile",
+    "stock_officers",
+    "stock_tailing_eps",
+    "stock_earning_calendar",
+    "stock_statement",
+    "stock_prices",
+    "stock_dividend_events",
+    "stock_split_events",
+    "exchange_rate",
+    "daily_treasury_yield",
+    "stock_earning_call_transcripts",
+    "stock_news",
+    "stock_revenue_breakdown",
+    "stock_shares_outstanding",
+    "stock_sec_filing",
+]
+
+EXTRA_FILES = [
+    ("data/company_tickers.json", "company_tickers.json"),
+]
+
+
+def download(url: str, dest: Path, label: str) -> None:
+    if dest.exists():
+        print(f"  skip  {label}  ({dest.stat().st_size / 1e6:.1f} MB on disk)")
+        return
+    print(f"  fetch {label} ...", end="", flush=True)
+    with requests.get(url, stream=True, timeout=60) as r:
+        r.raise_for_status()
+        tmp = dest.with_suffix(".tmp")
+        with open(tmp, "wb") as f:
+            for chunk in r.iter_content(chunk_size=8 * 1024 * 1024):
+                f.write(chunk)
+        tmp.rename(dest)
+    print(f"  {dest.stat().st_size / 1e6:.1f} MB")
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--out", default="data/parquet", help="local output directory")
+    args = parser.parse_args()
+
+    out = Path(args.out)
+    out.mkdir(parents=True, exist_ok=True)
+    print(f"Saving to: {out.resolve()}\n")
+
+    for table in PARQUET_TABLES:
+        url = f"{BASE}/data/{table}.parquet"
+        download(url, out / f"{table}.parquet", table)
+
+    for remote_path, local_name in EXTRA_FILES:
+        url = f"{BASE}/{remote_path}"
+        download(url, out / local_name, local_name)
+
+    print(f"\nDone. {sum(1 for _ in out.iterdir())} files in {out.resolve()}")
+
+
+if __name__ == "__main__":
+    main()