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/ast/block.rs

@@ -17,6 +17,10 @@ pub enum BlockNode {
     DefinitionList(DefinitionListNode),
     Table(TableNode),
     Comment(CommentNode),
+    /// Vimwiki tag line — `:tag1:tag2:`. Phase 12 (v1.1). The placement
+    /// rule (file / heading / standalone) is captured in `TagScope` so
+    /// LSP handlers and renderers can treat the three differently.
+    Tag(TagNode),
     Error(ErrorNode),
 }
 
@@ -142,6 +146,27 @@ pub struct CommentNode {
     pub content: String,
 }
 
+/// Where a `TagNode` falls on the page, matching vimwiki's placement rules.
+///
+/// - `File`        — line 0 or 1 of the document; tags the entire page.
+/// - `Heading(i)`  — within two lines below the i-th `HeadingNode` in the
+///                   document's `children`; tags that heading.
+/// - `Standalone`  — anywhere else; the tag is its own anchor on the source
+///                   line.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum TagScope {
+    File,
+    Heading(usize),
+    Standalone,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TagNode {
+    pub span: Span,
+    pub tags: Vec<String>,
+    pub scope: TagScope,
+}
+
 /// Resilient parse-failure node — emitted instead of aborting the document.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ErrorNode {