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/index.rs

@@ -32,10 +32,16 @@ pub struct IndexedPage {
     /// Phase 12: every `TagNode` on the page. `tags_by_name` on the
     /// containing `WorkspaceIndex` is the reverse map.
     pub tags: Vec<TagInfo>,
-    /// Phase 16: `Some(date)` when this page is a diary entry — i.e. its
-    /// file lives under `<root>/<diary_rel_path>/` and its stem parses as
-    /// `YYYY-MM-DD`. Used by the diary navigation commands.
+    /// Phase 16: `Some(date)` when this page is a *daily* diary entry —
+    /// stem parses as `YYYY-MM-DD`. Equivalent to
+    /// `diary_period.and_then(|p| if let DiaryPeriod::Day(d) = p { Some(d) } else { None })`.
+    /// Kept as its own field for back-compat with prev/next/list callers
+    /// that pre-date the multi-frequency upgrade.
     pub diary_date: Option<DiaryDate>,
+    /// Cluster 4 (vimwiki parity): `Some(period)` for any diary entry
+    /// (daily, weekly, monthly, yearly). The stem must match the
+    /// canonical format for one of the four frequencies.
+    pub diary_period: Option<nuwiki_core::date::DiaryPeriod>,
 }
 
 /// One tag occurrence on a page. `name` is the bare tag string (no
@@ -118,8 +124,12 @@ impl WorkspaceIndex {
     pub fn upsert(&mut self, uri: Url, ast: &DocumentNode) {
         self.remove(&uri);
         let name = page_name_from_uri(&uri, self.root.as_deref());
-        let diary_date =
-            diary_date_for_uri(&uri, self.root.as_deref(), self.diary_rel_path.as_deref());
+        let diary_period =
+            diary_period_for_uri(&uri, self.root.as_deref(), self.diary_rel_path.as_deref());
+        let diary_date = diary_period.and_then(|p| match p {
+            nuwiki_core::date::DiaryPeriod::Day(d) => Some(d),
+            _ => None,
+        });
         let mut page = IndexedPage {
             uri: uri.clone(),
             name: name.clone(),
@@ -128,6 +138,7 @@ impl WorkspaceIndex {
             outgoing: Vec::new(),
             tags: Vec::new(),
             diary_date,
+            diary_period,
         };
         index_blocks(&ast.children, &mut page);
 
@@ -224,19 +235,40 @@ impl WorkspaceIndex {
     }
 }
 
-/// Return a parsed [`DiaryDate`] when `uri` is a diary entry — its file
-/// path is under `<root>/<diary_rel_path>/` and its stem parses as
-/// `YYYY-MM-DD`. Anything else returns `None`.
+/// Return a parsed [`DiaryDate`] when `uri` is a daily diary entry —
+/// the path lives under `<root>/<diary_rel_path>/` and its stem parses
+/// as `YYYY-MM-DD`. For weekly/monthly/yearly entries see
+/// [`diary_period_for_uri`].
 pub fn diary_date_for_uri(
     uri: &Url,
     root: Option<&Path>,
     diary_rel_path: Option<&str>,
 ) -> Option<DiaryDate> {
+    match diary_period_for_uri(uri, root, diary_rel_path)? {
+        nuwiki_core::date::DiaryPeriod::Day(d) => Some(d),
+        _ => None,
+    }
+}
+
+/// Return a parsed [`DiaryPeriod`] when `uri` is a diary entry at any of
+/// the four supported frequencies. Recognises:
+///
+///   `YYYY-MM-DD`    → Day
+///   `YYYY-Www`      → Week (ISO)
+///   `YYYY-MM`       → Month
+///   `YYYY`          → Year
+///
+/// As with `diary_date_for_uri`, the path must sit under
+/// `<root>/<diary_rel_path>/` to be accepted; without a `root` we accept
+/// any stem (so ad-hoc test setups still work).
+pub fn diary_period_for_uri(
+    uri: &Url,
+    root: Option<&Path>,
+    diary_rel_path: Option<&str>,
+) -> Option<nuwiki_core::date::DiaryPeriod> {
     let path = uri.to_file_path().ok()?;
     let stem = path.file_stem()?.to_str()?;
-    let parsed = DiaryDate::parse(stem)?;
-    // Without a root we can't enforce the subdir requirement — accept any
-    // file whose stem looks like a date so that ad-hoc test setups work.
+    let parsed = nuwiki_core::date::DiaryPeriod::parse(stem)?;
     let Some(root) = root else {
         return Some(parsed);
     };