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/src/config.rs

@@ -41,6 +41,14 @@ pub struct WikiConfig {
     /// diary commands use. `daily` is supported; the others fall through
     /// to vimwiki-style names but the commands are no-ops for them.
     pub diary_frequency: String,
+    /// Weekly-diary naming: `iso` (nuwiki default — `YYYY-Www` labels,
+    /// always Monday-based) or `date`/`vimwiki` (the week-start day's date
+    /// `YYYY-MM-DD`, honouring `diary_start_week_day`). Only affects
+    /// `diary_frequency = weekly`.
+    pub diary_weekly_style: String,
+    /// Weekday the diary week begins on (`monday`..`sunday`, vimwiki's
+    /// `diary_start_week_day`). Used only when `diary_weekly_style = date`.
+    pub diary_start_week_day: String,
     /// Caption level for the diary index page headings (1 = h1).
     pub diary_caption_level: u8,
     /// Newest-first (`desc`) or oldest-first (`asc`) in the diary
@@ -174,6 +182,8 @@ impl WikiConfig {
             diary_rel_path: d.diary_rel_path,
             diary_index: d.diary_index,
             diary_frequency: d.diary_frequency,
+            diary_weekly_style: d.diary_weekly_style,
+            diary_start_week_day: d.diary_start_week_day,
             diary_caption_level: d.diary_caption_level,
             diary_sort: d.diary_sort,
             diary_header: d.diary_header,
@@ -201,6 +211,8 @@ impl WikiConfig {
             diary_rel_path: d.diary_rel_path,
             diary_index: d.diary_index,
             diary_frequency: d.diary_frequency,
+            diary_weekly_style: d.diary_weekly_style,
+            diary_start_week_day: d.diary_start_week_day,
             diary_caption_level: d.diary_caption_level,
             diary_sort: d.diary_sort,
             diary_header: d.diary_header,
@@ -245,6 +257,16 @@ impl WikiConfig {
     pub fn frequency(&self) -> nuwiki_core::date::DiaryFrequency {
         nuwiki_core::date::DiaryFrequency::parse(&self.diary_frequency)
     }
+
+    /// The diary navigation policy (frequency + weekly naming/week-start),
+    /// used by the diary commands to compute today / next / prev.
+    pub fn diary_calendar(&self) -> nuwiki_core::date::DiaryCalendar {
+        nuwiki_core::date::DiaryCalendar::new(
+            self.frequency(),
+            nuwiki_core::date::WeeklyStyle::parse(&self.diary_weekly_style),
+            nuwiki_core::date::WeekStart::parse(&self.diary_start_week_day),
+        )
+    }
 }
 
 fn default_diary_rel_path() -> String {
@@ -263,6 +285,15 @@ fn default_diary_frequency() -> String {
     "daily".to_string()
 }
 
+fn default_diary_weekly_style() -> String {
+    // nuwiki's original ISO-week labels; opt into `date` for vimwiki parity.
+    "iso".to_string()
+}
+
+fn default_diary_start_week_day() -> String {
+    "monday".to_string()
+}
+
 fn default_diary_sort() -> String {
     "desc".to_string()
 }
@@ -288,6 +319,8 @@ fn wiki_defaults() -> WikiDefaults {
         diary_rel_path: default_diary_rel_path(),
         diary_index: default_diary_index(),
         diary_frequency: default_diary_frequency(),
+        diary_weekly_style: default_diary_weekly_style(),
+        diary_start_week_day: default_diary_start_week_day(),
         // Match upstream vimwiki's `diary_caption_level` default of 0 (year
         // captions at the top level, months one below). Users override per-wiki.
         diary_caption_level: 0,
@@ -306,6 +339,8 @@ struct WikiDefaults {
     diary_rel_path: String,
     diary_index: String,
     diary_frequency: String,
+    diary_weekly_style: String,
+    diary_start_week_day: String,
     diary_caption_level: u8,
     diary_sort: String,
     diary_header: String,
@@ -519,6 +554,10 @@ struct RawWiki {
     #[serde(default)]
     diary_frequency: Option<String>,
     #[serde(default)]
+    diary_weekly_style: Option<String>,
+    #[serde(default)]
+    diary_start_week_day: Option<String>,
+    #[serde(default)]
     diary_caption_level: Option<u8>,
     #[serde(default)]
     diary_sort: Option<String>,
@@ -584,6 +623,8 @@ impl From<RawWiki> for WikiConfig {
             diary_rel_path: r.diary_rel_path.unwrap_or(d.diary_rel_path),
             diary_index: r.diary_index.unwrap_or(d.diary_index),
             diary_frequency: r.diary_frequency.unwrap_or(d.diary_frequency),
+            diary_weekly_style: r.diary_weekly_style.unwrap_or(d.diary_weekly_style),
+            diary_start_week_day: r.diary_start_week_day.unwrap_or(d.diary_start_week_day),
             diary_caption_level: r.diary_caption_level.unwrap_or(d.diary_caption_level),
             diary_sort: r.diary_sort.unwrap_or(d.diary_sort),
             diary_header: r.diary_header.unwrap_or(d.diary_header),