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>

crates/nuwiki-lsp/tests/index_and_config.rs

@@ -284,6 +284,8 @@ fn raw_wiki_parses_every_new_key() {
             "root": "/tmp/w",
             "index": "home",
             "diary_frequency": "weekly",
+            "diary_weekly_style": "date",
+            "diary_start_week_day": "sunday",
             "diary_caption_level": 2,
             "diary_sort": "asc",
             "diary_header": "Journal",
@@ -297,6 +299,8 @@ fn raw_wiki_parses_every_new_key() {
     let w = &cfg.wikis[0];
     assert_eq!(w.index, "home");
     assert_eq!(w.diary_frequency, "weekly");
+    assert_eq!(w.diary_weekly_style, "date");
+    assert_eq!(w.diary_start_week_day, "sunday");
     assert_eq!(w.diary_caption_level, 2);
     assert_eq!(w.diary_sort, "asc");
     assert_eq!(w.diary_header, "Journal");
@@ -305,6 +309,23 @@ fn raw_wiki_parses_every_new_key() {
     assert_eq!(w.list_margin, 2);
     assert_eq!(w.links_space_char, "_");
     assert!(w.auto_toc);
+
+    // The parsed keys drive a date-mode, Sunday-start diary calendar:
+    // a weekly note is the week-start date (YYYY-MM-DD), stepping ±7 days.
+    use nuwiki_core::date::{DiaryDate, DiaryPeriod};
+    let cal = w.diary_calendar();
+    let wed = DiaryPeriod::Day(DiaryDate::from_ymd(2026, 5, 27).unwrap());
+    assert_eq!(
+        cal.next(wed),
+        DiaryPeriod::Day(DiaryDate::from_ymd(2026, 5, 31).unwrap())
+    );
+}
+
+#[test]
+fn diary_calendar_defaults_to_iso_weeks() {
+    let cfg = WikiConfig::empty();
+    assert_eq!(cfg.diary_weekly_style, "iso");
+    assert_eq!(cfg.diary_start_week_day, "monday");
 }
 
 #[test]