feat(SI): add school holidays for 2026/2027

[tadejzunko]

src/si/holidays/holidays.school.csv stops at the 2025/2026 summer break, so a query for any date in the 2026/2027 school year comes back empty. The ministry settled that year's calendar in February (primary) and April (secondary) 2026.

These 9 rows complete it.

Start	End	Holiday	Scope
2026-10-26	2026-10-30	Jesenske počitnice	national
2026-12-25	2027-01-02	Novoletne počitnice	national, primary
2026-12-28	2026-12-31	Novoletne počitnice	national, secondary
2027-02-15	2027-02-19	Zimske počitnice	VR
2027-02-22	2027-02-26	Zimske počitnice	ZR
2027-04-26	—	Pouka prost dan	national
2027-04-27	2027-05-02	Prvomajske počitnice	national, primary
2027-04-28	2027-04-30	Prvomajske počitnice	national, secondary
2027-06-28	2027-08-31	Poletne počitnice	national

Sources

Both are ministerial instructions issued under the respective Pravilnik o šolskem koledarju:

• Primary — Podrobnejša navodila o šolskem koledarju za osnovne šole za šolsko leto 2026/2027, no. 6034-11/2026-3350-1, 5 Feb 2026 (index)
• Secondary — Počitnice in pouka prosti dnevi … za šolsko leto 2026/2027, no. 6036-181/2026-3350-1, 23 Apr 2026 (index)

Every date is read off those two documents. None is carried forward from a previous year, which matters because two of them break the usual pattern:

• Summer starts on the 28th, not the 26th. Slovenian summer holidays have begun on 26 June for years, but 26 June 2027 is a Saturday, so the ministry set the Monday.
• The winter regions are the other way round from last year. For 2026/2027 VR runs 15–19 February and ZR runs 22–26; in 2025/2026 it was ZR first. The two halves alternate annually, so anyone extrapolating from the existing rows would put both regions on the wrong week.

One caveat about Kočevje

The two documents disagree on it. The secondary-school calendar excludes Kočevje from the 15–19 February group along with Ribnica, Sodražica, Loški Potok, Osilnica and Kostel; the primary-school calendar lists those five but not Kočevje. So Kočevje's primary schools take the earlier week and its secondary schools the later one.

subdivisions.csv maps Kočevje to ZR, which matches the secondary reading. A single VR/ZR tag per municipality can't express a split that depends on school type, so I've left the mapping alone and these rows follow it — flagging it rather than working around it, since it predates this change and affects one municipality.

Checks

bin/test_all_tables.py passes on the modified tree. The 9 UUIDs are freshly generated and don't collide with any of the 14,061 IDs already in the repository. Rows are appended in date order, the file keeps its BOM and LF endings, and all six holiday names already exist in the file, so no new name strings are introduced.