feat(diary): restore diary_start_week_day via configurable weekly naming

c63ec67 dropped diary_start_week_day, hardwiring the weekly diary to ISO
(Monday) weeks. Upstream vimwiki instead names a weekly note by the
week-start day's date (YYYY-MM-DD) and honours diary_start_week_day. Rather
than force one scheme, make it a per-wiki choice so migrators keep upstream
behaviour while existing nuwiki weekly files keep working:

- New per-wiki key `diary_weekly_style`: `iso` (default — `YYYY-Www`,
  Monday-based, nuwiki's original) or `date`/`vimwiki` (`YYYY-MM-DD` of the
  week-start day, upstream parity).
- Restored per-wiki key `diary_start_week_day` (`monday`..`sunday`, default
  monday); applies only in `date` mode.

Implementation:
- nuwiki-core::date gains WeeklyStyle, WeekStart, and DiaryCalendar (owns
  today/next/prev — date-mode snaps to the week-start and steps ±7 days;
  iso/daily/monthly/yearly defer to the existing DiaryPeriod logic).
- WikiConfig gains the two fields (+ defaults, RawWiki, From) and a
  diary_calendar() builder; commands.rs diary_open_relative and the
  next/prev pivot use it.

Defaults preserve current behaviour (iso/monday), so no breaking change.
Tests: DiaryCalendar cases in nuwiki-core/tests/diary.rs (snap, ±7, sunday
start, iso delegation, daily) + config round-trip in
nuwiki-lsp/tests/index_and_config.rs. README + doc/nuwiki.txt + lua config
comment updated. fmt + clippy clean; all crate tests + config-parity green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

development/vimwiki-gap.md

@@ -109,13 +109,18 @@ fix site.
   upstream `0`. _Fix:_ `wiki_defaults()` in `crates/nuwiki-lsp/src/config.rs` now
   defaults `diary_caption_level: 0` (year captions top-level, months one below),
   still per-wiki overridable. Test `defaults_carry_vimwiki_per_wiki_keys` updated.
-- [ ] **`diary_start_week_day`** — **wrongly removed** in `c63ec67`. The removal
-  treated it as a server concern and hardwired the weekly diary to ISO-Monday
-  (`crates/nuwiki-core/src/date.rs` `monday_of_iso_week`), but choosing which
-  weekday the diary week begins on is a **client-side concern** and it **matters
-  for parity** — non-Monday users lose the upstream option entirely. _Fix:_
-  restore the config key client-side (`lua/nuwiki/config.lua`) and thread the
-  week-start through to the diary date logic instead of hardcoding Monday.
+- [x] **`diary_start_week_day`** — removed in `c63ec67` (weekly diary hardwired
+  to ISO-Monday). Restored as a **configurable choice** so migrators keep
+  upstream behaviour while new wikis can use ISO weeks. New per-wiki key
+  `diary_weekly_style`: `iso` (default — `YYYY-Www`, Monday-based, nuwiki's
+  original) or `date`/`vimwiki` (`YYYY-MM-DD` of the week-start day, upstream
+  parity, honouring the restored `diary_start_week_day` = `monday`..`sunday`).
+  _Impl:_ `nuwiki_core::date::{WeeklyStyle, WeekStart, DiaryCalendar}` own the
+  today/next/prev logic; `WikiConfig::diary_calendar()` builds it from config;
+  the diary commands (`commands.rs` `diary_open_relative` + next/prev pivot) use
+  it. Defaults keep existing nuwiki weekly files working (no breaking change).
+  Covered by calendar tests in `crates/nuwiki-core/tests/diary.rs` and the
+  config round-trip in `crates/nuwiki-lsp/tests/index_and_config.rs`.
 
 ## P3 — Niche / low impact