phase 6: LSP foundation — didOpen/didChange, diagnostics, outline

`tower-lsp` server (`nuwiki-lsp::run` + `run_stdio`) wired up by the
`nuwiki-ls` binary. Backend holds an `Arc<DashMap<Url, DocumentState>>`
per SPEC §6.10 plus an `Arc<SyntaxRegistry>` pre-loaded with
`VimwikiSyntax`. Re-parse is full per change; incremental parsing
deferred post-v1 per spec.

Capabilities declared:
- `text_document_sync = FULL`
- `document_symbol_provider = true`
- `position_encoding = UTF-8` when the client opts into LSP 3.17+
  encodings (SPEC §6.11). Otherwise we keep the legacy UTF-16 default
  and translate spans through a per-line UTF-16 code-unit walk that
  handles BMP and surrogate pairs alike.

Handlers: `initialize` (negotiate encoding), `initialized` (log it),
`shutdown`, `did_open` / `did_change` (full re-parse +
`publishDiagnostics`), `did_close` (drop state, clear diagnostics),
`document_symbol` (nested outline).

Diagnostics: every `BlockNode::Error` (from §6.7's resilient parser)
emits one `DiagnosticSeverity::ERROR` with `source = "nuwiki"`. The
walker descends into Blockquotes and ListItems so errors nested inside
those don't slip through.

Document outline: nested `DocumentSymbol` tree built recursively from
heading levels (`= H1 =` → root, `== H2 ==` → child of preceding H1,
etc.). Titles flatten inline markers via `inline_to_text` so
`*world*` shows up as `world` in the outline.

Pure helpers (`to_lsp_position`, `to_lsp_range`, `ast_diagnostics`,
`headings_to_symbols`) are `pub` so the integration tests can verify
conversion correctness without spinning up the async server. End-to-
end LSP message exercises will land in Phase 8 alongside navigation.

Tests (11): UTF-8 passthrough, UTF-16 conversion for é + 🦀, single
and multi-error diagnostic emission, blockquote descent, flat outline,
nested outline by levels, inline-marker stripping in titles, empty-
heading fallback, registry-dispatch parity.

Dep pin: `idna_adapter` held at 1.2.0 — 1.2.2 needs Cargo's
edition2024 feature (Rust 1.85), our MSRV is 1.83.

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

README.md

@@ -3,8 +3,9 @@
 A Vim/Neovim plugin providing full vimwiki syntax support, implemented as a
 Rust-based language server (LSP).
 
-> **Status:** Phase 5 (HTML renderer) complete. End-to-end source → AST → HTML
-> works via `VimwikiSyntax` + `HtmlRenderer`. LSP and editor glue still pending.
+> **Status:** Phase 6 (LSP foundation) complete. `nuwiki-ls` speaks LSP over
+> stdio: didOpen/didChange/didClose, parse diagnostics, nested document
+> outline. Editor glue, semantic tokens, and navigation still pending.
 
 See [`SPEC.md`](./SPEC.md) for the full project specification.
 
@@ -18,8 +19,8 @@ See [`SPEC.md`](./SPEC.md) for the full project specification.
 | 3 | Vimwiki Lexer | ✅ done |
 | 4 | Vimwiki Parser | ✅ done |
 | 5 | Renderer | ✅ done |
-| 6 | LSP Foundation | ⏳ next |
-| 7 | Semantic Tokens | ⏳ |
+| 6 | LSP Foundation | ✅ done |
+| 7 | Semantic Tokens | ⏳ next |
 | 8 | Navigation | ⏳ |
 | 9 | Editor Glue | ⏳ |
 | 10 | CI/CD release pipeline | ⏳ |