phase 16: diary path conventions + nav commands

Hand-rolled date primitive (no chrono/time dep) plus the diary
command surface that closes the `:VimwikiMakeDiaryNote` ergonomics gap.

nuwiki-core/src/date.rs:
- `DiaryDate { year, month, day }` with strict `YYYY-MM-DD` parsing
  (rejects missing zero-padding, alternate separators, whitespace,
  impossible calendar dates including the leap-year exceptions).
- `next_day` / `prev_day` via Howard Hinnant's days-from-civil
  algorithm — proleptic Gregorian, walks across month/year/leap-day
  boundaries.
- `today_utc()` from `SystemTime::now()`. UTC-only by design: a
  local-tz implementation would need a tz database we don't ship.
- `Ord` by epoch days, `Display`/`format` round-trip with `parse`.

WikiConfig (P15 resolution):
- `diary_rel_path: String` (default `"diary"`) — mirrors vimwiki's
  `g:vimwiki_diary_rel_path`.
- `diary_index: String` (default `"diary"`) — the stem inside the
  diary subdir.
- `diary_dir()`, `diary_index_path()`, `diary_path_for(date)` —
  centralise the path math so commands stay declarative.
- `RawWiki` (init-options wire format) accepts both fields, so users
  with `g:vimwiki_diary_rel_path = "journal"` migrate cleanly.

WorkspaceIndex:
- New `diary_rel_path: Option<String>` field set by `Wiki::new`.
- `IndexedPage.diary_date: Option<DiaryDate>` — populated in `upsert`
  when the URI is under `<root>/<diary_rel_path>/` and the stem
  parses. `diary_date_for_uri` is the standalone classifier.
- The classifier *requires* the URI to be inside the diary subdir
  when a root is known; with no root it accepts any file whose stem
  is a date (kept for the ad-hoc test/scratch case).

nuwiki-lsp/src/diary.rs:
- `uri_for_date` / `index_uri` — bridge from `DiaryDate` and config
  to `file://` URIs.
- `list_entries(index) → Vec<DiaryEntry>` — ascending, plus
  `list_entries_filtered(year, month)` for calendar hooks.
- `next_entry` / `prev_entry` — strict less-than/greater-than
  comparison so navigating from a non-diary page (uses today as
  pivot) still moves to the right entry.
- `build_index_body` — newest-first, grouped under `== YYYY ==` /
  `=== Month ===` subheadings, emits `- [[diary/YYYY-MM-DD]]`. Matches
  `:VimwikiDiaryGenerateLinks`.
- `DiaryEntry` JSON shape: `{ date: "YYYY-MM-DD", uri }`. Manual
  serde impl so DiaryDate stays serde-free in nuwiki-core.

Commands (9 new):
- `nuwiki.diary.openToday` / `openYesterday` / `openTomorrow` →
  `{ uri, date }`.
- `nuwiki.diary.openIndex` → `{ uri }`.
- `nuwiki.diary.generateIndex` → WorkspaceEdit. Three paths:
  live document in `backend.documents` → full-doc replace; on-disk
  index → read + full-doc replace; missing → CreateFile + insert.
- `nuwiki.diary.next` / `prev` → pivots on the current page's
  `diary_date`, falls back to today.
- `nuwiki.diary.listEntries` → optional `year` / `month` filter.
- `nuwiki.diary.openForDate` → strict `YYYY-MM-DD` arg.

All commands accept an optional `uri` to scope to a particular wiki
when multi-wiki lands; the dispatcher falls back to `default_wiki()`.

Tests: 35 new in `phase16_diary.rs` (date parser exhaustively,
calendar arithmetic across boundaries + leaps, path conventions,
upsert classification including the "date filename outside diary dir"
no-op case, list/filter/nav, index body grouping, serde shape, command
list completeness). Total 309 tests pass.

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

crates/nuwiki-lsp/src/index.rs

@@ -17,6 +17,7 @@ use nuwiki_core::ast::{
     BlockNode, BlockquoteNode, DocumentNode, InlineNode, LinkKind, LinkTarget, ListNode, Span,
     TableNode, TagScope,
 };
+use nuwiki_core::date::DiaryDate;
 use tower_lsp::lsp_types::Url;
 
 #[derive(Debug, Clone)]
@@ -31,6 +32,10 @@ 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.
+    pub diary_date: Option<DiaryDate>,
 }
 
 /// One tag occurrence on a page. `name` is the bare tag string (no
@@ -81,6 +86,10 @@ pub struct TagOccurrence {
 #[derive(Debug, Default)]
 pub struct WorkspaceIndex {
     pub root: Option<PathBuf>,
+    /// Phase 16: subdir relative to `root` that holds diary entries.
+    /// Mirrors `WikiConfig::diary_rel_path`; stored here so `upsert` can
+    /// classify each indexed URI without a back-reference to the config.
+    pub diary_rel_path: Option<String>,
     pub pages_by_uri: HashMap<Url, IndexedPage>,
     pub pages_by_name: HashMap<String, Url>,
     pub backlinks: HashMap<String, Vec<Backlink>>,
@@ -98,12 +107,19 @@ impl WorkspaceIndex {
         }
     }
 
+    pub fn with_diary_rel_path(mut self, rel: Option<String>) -> Self {
+        self.diary_rel_path = rel;
+        self
+    }
+
     /// Insert or update an indexed page. Removes any prior indexing for
     /// the same URI first (handles renames and re-parses without leaking
     /// stale backlinks).
     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 mut page = IndexedPage {
             uri: uri.clone(),
             name: name.clone(),
@@ -111,6 +127,7 @@ impl WorkspaceIndex {
             headings: Vec::new(),
             outgoing: Vec::new(),
             tags: Vec::new(),
+            diary_date,
         };
         index_blocks(&ast.children, &mut page);
 
@@ -207,6 +224,31 @@ 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`.
+pub fn diary_date_for_uri(
+    uri: &Url,
+    root: Option<&Path>,
+    diary_rel_path: Option<&str>,
+) -> Option<DiaryDate> {
+    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 Some(root) = root else {
+        return Some(parsed);
+    };
+    let parent = path.parent()?;
+    let expected = root.join(diary_rel_path.unwrap_or("diary"));
+    if parent.starts_with(&expected) {
+        Some(parsed)
+    } else {
+        None
+    }
+}
+
 /// Derive a page name from a `file://` URL. With a workspace root the name
 /// is the URL's path relative to root, sans `.wiki`. Without a root it's
 /// just the filename stem.