phase 8: navigation — definition, refs, hover, completion, ws/symbol

P8 resolved (SPEC §11): lazy + background indexing with progress.
Initial scan runs in a tokio task spawned from `initialized`; the
server stays responsive immediately and nav features answer with
partial data until the index is complete.

WorkspaceIndex (`index.rs`):
- Per-URI `IndexedPage` keeps name, title, headings, outgoing links.
- Reverse maps: `pages_by_name`, `backlinks: HashMap<page, …>`.
- `upsert` scrubs stale entries first so re-parses don't leak.
- `resolve` handles Wiki + AnchorOnly link kinds against the index;
  other kinds (Interwiki/Diary/File/Local/Raw) fall through.
- `slugify` for anchors (lowercases, collapses runs, drops trailing
  dashes, preserves Unicode), `page_name_from_uri` for workspace-
  rooted names, `walk_wiki_files` for the background scanner.

Backend wiring (`lib.rs`):
- `initialize` captures workspace root from `workspaceFolders` or the
  deprecated `root_uri`; advertises definition/references/hover/
  completion (trigger `[`)/workspace_symbol providers.
- `initialized` spawns the indexer, wrapped in
  `window/workDoneProgress` begin/end notifications.
- `did_open`/`did_change` synchronously update the index alongside
  publishing diagnostics; `did_close` keeps the indexed projection
  (file may still exist on disk and other pages link to it) and
  only clears diagnostics + drops the live document state.

Position lookup (`nav.rs`):
- `lsp_to_byte_pos` translates between LSP encoding (UTF-8 or
  UTF-16) and our byte columns.
- `find_inline_at` descends blocks → inlines → inline containers,
  returning the most specific inline whose span covers the cursor.

Handlers:
- definition: wikilinks resolve to a target URI; anchored links
  return the range at the matching heading.
- references: backlinks of the cursor's link target, or of the
  current page when the cursor isn't on a link. Ranges fall back
  to byte-column LSP positions when the source doc isn't open.
- hover: markdown preview with page title + outline (first 8
  headings), or a "not in index" fallback.
- completion: fires only inside an unterminated `[[ … ` on the
  current line; suggests every indexed page name.
- workspace/symbol: case-insensitive substring over every indexed
  heading.

Tests (20 new): slugify (basic / punctuation / Unicode / trailing
dash), page-name derivation under various roots, upsert / replace /
remove, resolve for Wiki + AnchorOnly, sorted `page_names`, UTF-8
passthrough + UTF-16 conversion, position lookup hits and misses.

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

README.md

@@ -3,10 +3,10 @@
 A Vim/Neovim plugin providing full vimwiki syntax support, implemented as a
 Rust-based language server (LSP).
 
-> **Status:** Phase 7 (semantic tokens) complete. The server now serves
-> `semanticTokens/full` + `/range` with custom `vimwiki*` token types and
-> `level1`..`level6` + `centered` modifiers. Navigation features and
-> editor glue still pending.
+> **Status:** Phase 8 (navigation) complete. The server now serves
+> definition, references, hover, completion, `workspace/symbol`, plus a
+> background-indexed workspace per P8. Only editor glue and the release
+> pipeline are still pending.
 
 See [`SPEC.md`](./SPEC.md) for the full project specification.
 
@@ -22,8 +22,8 @@ See [`SPEC.md`](./SPEC.md) for the full project specification.
 | 5 | Renderer | ✅ done |
 | 6 | LSP Foundation | ✅ done |
 | 7 | Semantic Tokens | ✅ done |
-| 8 | Navigation | ⏳ next |
-| 9 | Editor Glue | ⏳ |
+| 8 | Navigation | ✅ done |
+| 9 | Editor Glue | ⏳ next |
 | 10 | CI/CD release pipeline | ⏳ |
 
 ## Repository layout