feat(KR): add substitute holiday (대체공휴일) support

[g1mn]

Summary

• Add substitute-day (대체공휴일) rules for Korea: Children's Day, Independence Movement Day, Liberation Day, National Foundation Day, Hangul Day, Buddha's Birthday, Christmas Day and Constitution Day, each active from the year its substitute rule took legal effect (2014 / 2021 / 2023 / 2026 depending on the holiday).
• Seollal and Chuseok get explicit dated entries instead of a generic rule: their 3-day block can be pushed by either a Sunday overlap or an overlap with another public holiday (e.g. Chuseok colliding with National Foundation Day), and the existing korean 01-0-01 P3D rule for Seollal is anchored one day later than the real 전날/당일/다음날 window, so a rule computed against it directly would land on the wrong date. Entries are computed from the corrected window and cover 2014-2050 (see comment in the yaml for details/limitations).
• Verified against known real-world substitute dates, e.g. 2018 (no Seollal substitute), 2024-02-12 (Seollal), 2025-05-06 (Children's Day/Buddha's Birthday collision), 2017-10-06 (Chuseok/National Foundation Day collision).

Test plan

• npm run build
• npm test (regenerated test/fixtures/KR-*.json, full suite passing: 5373 tests)
• Cross-checked computed substitute dates for 2014-2050 against public sources for several representative years

Closes #640

[commenthol]

Many thanks.