parity(4): diary weekly / monthly / yearly frequency support

vimwiki's `diary_frequency` config has been honoured by the existing
`nuwiki.diary.openToday` / `openYesterday` / `openTomorrow` commands
end-to-end. Setting `diary_frequency = 'weekly'` (or `monthly` /
`yearly`) now creates and addresses entries at that cadence — file
stems follow the canonical formats:

  daily    YYYY-MM-DD     2026-05-12.wiki
  weekly   YYYY-Www       2026-W19.wiki      (ISO 8601 week)
  monthly  YYYY-MM        2026-05.wiki
  yearly   YYYY           2026.wiki

Core additions (nuwiki_core::date):
  - DiaryFrequency enum + permissive `parse` (unknown → Daily)
  - DiaryPeriod enum unifying Day / Week / Month / Year
  - format / parse / next / prev / today_utc + first_day, with
    proper ISO-week math (Thursday rule, year-boundary handling)

LSP wiring:
  - WikiConfig::frequency() and diary_path_for_period()
  - crate::diary::uri_for_period
  - Index now records `diary_period` for any of the four flavours;
    `diary_date` is preserved (filtered to Day-only) for back-compat
    with existing daily-only callers
  - `nuwiki.diary.next` / `nuwiki.diary.prev` now navigate at the
    *same flavour* as the current entry (or at the wiki's configured
    frequency when off a diary page), via new
    `crate::diary::next_period` / `prev_period` helpers
  - `nuwiki.diary.openToday` returns the period stem + frequency in
    its response payload alongside the URI

Tests:
  - 12 in crates/nuwiki-core/tests/diary_period.rs covering the date
    math (ISO week boundaries, format round-trips, next/prev across
    year edges, etc.)
  - 9 in crates/nuwiki-lsp/tests/diary_frequency.rs covering
    WikiConfig path computation, index recognition for all four
    stems, period navigation, and diary-dir filtering

Gates: 450 Rust / 39 Neovim / 12 Vim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/config.rs

@@ -232,6 +232,20 @@ impl WikiConfig {
         p.push(format!("{}{}", date.format(), self.file_extension));
         p
     }
+
+    /// Absolute path for a diary entry covering `period`. Honours the
+    /// wiki's `diary_frequency` indirectly — callers compute the period
+    /// at the right cadence and we just pick the stem from its format.
+    pub fn diary_path_for_period(&self, period: &nuwiki_core::date::DiaryPeriod) -> PathBuf {
+        let mut p = self.diary_dir();
+        p.push(format!("{}{}", period.format(), self.file_extension));
+        p
+    }
+
+    /// `DiaryFrequency` parsed from this wiki's `diary_frequency` field.
+    pub fn frequency(&self) -> nuwiki_core::date::DiaryFrequency {
+        nuwiki_core::date::DiaryFrequency::parse(&self.diary_frequency)
+    }
 }
 
 fn default_diary_rel_path() -> String {