[bravohex]

Vietnam's holiday data is missing several observances, uses inconsistent
Vietnamese names, and labels the existing five-day Tet holiday period as
New Year's Eve.

This PR updates the Vietnam calendar:

• Add 14 annual observances and Vietnam Culture Day on November 24,
  effective from 2026.
• Add a separate one-day New Year's Eve observance and correctly name the
  existing five-day public Tet holiday period. Document that actual leave
  dates may vary by annual announcement and employer arrangements.
• Standardize Vietnamese names and capitalization, correct “Điện Biên Phủ”,
  and update English translations where the meaning changes.
• Preserve “Ngày Sách Việt Nam” through 2021 and use
  “Ngày Sách và Văn hóa đọc Việt Nam” from 2022.
• Calculate Thanh Minh using the Vietnamese solar term and Hàn thực using
  the Vietnamese lunar calendar.
• Update Vietnam fixtures for 2015–2029, covered by the existing shared
  holiday test suite.

The 2026 calendar contains 43 records: 8 public and 35 observance records.
These are holiday records, not a count of distinct paid days off; the first
day of Tet remains within the five-day holiday period.

Changes are limited to VN.yaml and 15 Vietnam fixture files.
Organization-specific leave schedules, substitute days, and workday swaps
are outside the scope of this update.

Sources are cited beside the relevant rules using # @source, with the
existing Wikipedia # @attrib retained. Key references:

• Vietnam Culture Day — Resolution 28/2026/QH16
• Vietnam Book and Reading Culture Day — Decision 1862/QD-TTg