Bootstrap HK IPO historical archive

Request:
- Use the project archivist workflow to update historical IPO data.

Changes:
- Add an embedded SQLite archive at data/hk_ipo.sqlite.
- Add schema/hk_ipo.schema.sql and scripts/bootstrap_historical_data.py for reproducible archive generation.
- Archive HKEXnews source PDFs for 06658, 06675, and 06106 under repo-relative data/raw paths.
- Export Git-friendly snapshots for ipo_master, offering_terms, ipo_demand, source_refs, and data_gaps.
- Add .gitignore rules for Python cache and SQLite transient files.

Verification:
- Re-ran the bootstrap script successfully.
- Ran PRAGMA integrity_check on the SQLite database.
- Verified source_refs paths are repo-relative, files exist, and SHA-256 hashes match.
- Verified snapshot row counts match SQLite table counts.
- Ran git diff --check and searched generated durable files for machine-specific absolute paths.

data/snapshots/offering_terms.csv

@@ -0,0 +1,4 @@
+ticker,source_id,prospectus_date,offer_price_hkd,board_lot,min_subscription_amount_hkd,global_offer_shares,hk_offer_shares_initial,international_offer_shares_initial,public_offer_pct_initial,over_allotment_offer_shares,offer_size_adjustment_offer_shares,market_cap_hkd_m,gross_proceeds_hkd_m,net_proceeds_hkd_m,issued_shares_upon_listing,data_as_of
+06106,06106_prospectus_notice_2026_06_15,2026-06-15,101.6,50,5131.24,10497300,524900,9972400,0.05,1574550,1574550,,,,,2026-06-15T06:15:00Z
+06658,06658_prospectus_2026_06_05,2026-06-05,43.58,100,4401.96,11464100,1146500,10317600,0.1,,,3434.59,499.6,440.1,78811208,2026-06-15T06:15:00Z
+06675,06675_global_offering_announcement_2026_06_09,2026-06-09,18.36,200,3709.04,53407000,5340800,48066200,0.1,8011000,,6959.2,,906.7,379041820,2026-06-15T06:15:00Z