phase 12: tags — lexer, AST, index, anchor resolution, ws/symbol

Vimwiki `:tag1:tag2:` syntax across the full stack. P11 resolved as
strict-vimwiki — no markdown `#tag` alias.

AST (nuwiki-core):
- `TagScope { File, Heading(usize), Standalone }` mirrors vimwiki's
  three placement-determined meanings: lines 0-1 → File; one or two
  lines below the most recent heading → Heading(idx); otherwise
  Standalone.
- `TagNode { span, tags: Vec<String>, scope }` + `BlockNode::Tag`.
- `PageMetadata.tags: Vec<String>` accumulates the file-scope tag
  names — convenience projection of the Tag blocks whose scope is
  File. New field is additive (Default derive) so existing consumers
  using `..Default::default()` keep working.
- `Visitor::visit_tag` lands with a default body and `walk_block`
  dispatches the new arm.

Lexer (`syntax/vimwiki/lexer.rs`):
- `VimwikiTokenKind::Tag(Vec<String>)` block-level token.
- `try_lex_tag` runs in the normal-line dispatcher right after heading
  recognition; demands the *whole trimmed line* match `:name:name…:`,
  with non-empty whitespace-free names between adjacent colons. Empty
  segments (`::tag::`), whitespace inside a name, or single-colon
  lines are all rejected. Doesn't fight `Term::` (definition-list
  marker) because that pattern requires text before the `::`.
- Indentation is tolerated; the span covers the trimmed run.

Parser (`syntax/vimwiki/parser.rs`):
- `ParseState` gains `headings_seen` + `last_heading_line`.
- `parse_heading` bumps both on every closed heading so the tag
  parser can detect the heading window.
- `parse_tag` reads the Tag token, computes the scope via
  `scope_for_tag(tag_line)`, eats the trailing newline, and emits the
  TagNode.
- `parse_document` post-processes each block: when a Tag with
  `scope == File` is emitted, its names are pushed (de-duped) into
  `metadata.tags`.

Renderer (`render/html.rs`):
- `render_tag` emits `<div class="tags"><span class="tag"
  id="tag-name">name</span>...</div>`. The `id` is what makes
  `[[Page#tag-name]]` jump to the rendered tag.

Semantic tokens (`semantic_tokens.rs`):
- New `TOKEN_TAG = 19` / `vimwikiTag` so editors highlight tag lines
  distinctly. `emit_block` adds the Tag arm.

Workspace index (`index.rs`):
- `IndexedPage.tags: Vec<TagInfo>` records each tag occurrence on a
  page with its span + scope.
- `WorkspaceIndex.tags_by_name: HashMap<String, Vec<TagOccurrence>>`
  is the reverse map for cross-page anchor resolution + the Phase 13
  `nuwiki.tags.search` command.
- `upsert` populates both; `remove` scrubs the reverse map and drops
  empty entries; `tags_for(name)` is the public lookup.

LSP handlers (`lib.rs`):
- `heading_range_in` falls back to tag matching when the anchor
  isn't a heading slug — so `[[Page#some-tag]]` lands on the matching
  TagNode. Heading anchors win over tag anchors when both match (the
  vimwiki precedence).
- `workspace/symbol` includes tags alongside headings, named `:tag:`
  and kinded `PROPERTY` so editor UIs can distinguish them.

Tests (20 new):
- Syntax (15 in `vimwiki_tags.rs`):
  lexer recognition + rejection of `::tag::`, whitespace-in-name,
  Term::-conflict; parser scope paths (file, heading-1-below,
  heading-2-below, standalone, no-heading, line-1 priority);
  metadata dedup; renderer HTML + escaping.
- LSP index (5 in `tags_index_and_lsp.rs`):
  per-page tag population, cross-page `tags_by_name` aggregation,
  re-upsert replaces stale tags, `remove` cleans the reverse map,
  heading-scope tag indexed but not in file metadata.

All 191 v1.0 + Phase 11 tests still pass — backward compatible.

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

crates/nuwiki-core/src/syntax/vimwiki/lexer.rs

@@ -96,6 +96,12 @@ pub enum VimwikiTokenKind {
     CommentMultiClose,
     CommentMultiLine(String),
 
+    // ----- Tags (SPEC §12.3) -----
+    /// `:tag1:tag2:` — colon-delimited tag line. The lexer treats this as
+    /// block-level (must be the entire trimmed content of the line).
+    /// Scope (file / heading / standalone) is the parser's job.
+    Tag(Vec<String>),
+
     // ----- Page placeholders (SPEC §9) -----
     PlaceholderTitle(Option<String>),
     PlaceholderNohtml,
@@ -318,6 +324,7 @@ impl<'src> LexState<'src> {
             || self.try_lex_placeholder(line)
             || self.try_lex_horizontal_rule(line)
             || self.try_lex_heading(line)
+            || self.try_lex_tag(line)
             || self.try_lex_table_row(line)
             || self.try_lex_blockquote(line)
             || self.try_lex_list_item(line)
@@ -713,6 +720,44 @@ impl<'src> LexState<'src> {
         true
     }
 
+    /// `:tag1:tag2:tag3:` — colon-delimited tag line (block-level).
+    ///
+    /// The whole trimmed line must match: starts and ends with `:`, with
+    /// non-empty whitespace-free names between adjacent colons. Empty
+    /// segments or whitespace inside tag names disqualify the line.
+    /// Doesn't conflict with `::` (definition list marker) because that
+    /// pattern requires text *before* the `::`.
+    fn try_lex_tag(&mut self, line: &str) -> bool {
+        let trimmed = line.trim();
+        if trimmed.len() < 3 || !trimmed.starts_with(':') || !trimmed.ends_with(':') {
+            return false;
+        }
+        // `:tag1:tag2:` splits as `["", "tag1", "tag2", ""]`; the empty
+        // strings at the ends are expected, everything in between must be
+        // non-empty and contain no whitespace.
+        let parts: Vec<&str> = trimmed.split(':').collect();
+        if parts.first() != Some(&"") || parts.last() != Some(&"") {
+            return false;
+        }
+        let names = &parts[1..parts.len() - 1];
+        if names.is_empty()
+            || names
+                .iter()
+                .any(|t| t.is_empty() || t.chars().any(char::is_whitespace))
+        {
+            return false;
+        }
+        let indent = (line.len() - line.trim_start().len()) as u32;
+        let end_col = indent + trimmed.len() as u32;
+        self.emit(
+            VimwikiTokenKind::Tag(names.iter().map(|s| (*s).to_owned()).collect()),
+            indent,
+            end_col,
+        );
+        self.emit_newline(line);
+        true
+    }
+
     fn try_lex_definition_term(&mut self, line: &str) -> bool {
         // `Term:: Definition` or `Term::` (continuation lines follow).
         // The `::` must be preceded by non-`:` text and followed by either