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>

lua/nuwiki/config.lua

@@ -21,6 +21,7 @@ M.defaults = {
   -- `doc/nuwiki.txt`):
   --   name, root, file_extension, syntax, index
   --   diary_rel_path, diary_index, diary_frequency,
+  --   diary_weekly_style, diary_start_week_day,
   --   diary_caption_level, diary_sort, diary_header
   --   html_path, template_path, template_default, template_ext,
   --   template_date_format, css_name, auto_export, auto_toc,