phase 18: multi-wiki resolver + picker commands

The Phase 11 plumbing already landed the data structures (`Wiki`
aggregate, per-wiki `WorkspaceIndex`, `wikis = [...]` config shape
with v1.0 desugaring). Phase 18 closes the user-visible gap: cross-
wiki link resolution + the picker command surface.

Cross-wiki resolution:
- `Backend::wikis_snapshot` — clone the Vec under the read lock
  so commands can drop it before awaiting.
- `Backend::wiki_by_index` / `wiki_by_name` — pair with the
  parser's `wiki<N>:` / `wn.<Name>:` interwiki shapes.
- `Backend::resolve_target_uri(target, source_uri)` — single entry
  point for cross-wiki page lookup:
    - `Wiki` / `AnchorOnly` → source wiki's index (Phase 8
      behaviour preserved).
    - `Interwiki` → routes to the wiki referenced by
      `wiki_index` / `wiki_name`, then resolves the page in that
      wiki's `pages_by_name`. Returns `None` when no wiki matches
      or the page isn't indexed.
- `Backend::heading_range_for(target_uri, anchor)` — anchor lookup
  on the target wiki's index. Phase 8's `goto_definition` was
  computing this against the *source* wiki's index, which broke for
  interwiki anchors; both `goto_definition` and `hover` are
  refactored onto the new pair.

Commands:
- `nuwiki.wiki.listAll` — returns `[{ id, name, root, syntax,
  file_extension }]`. Drives the picker UI.
- `nuwiki.wiki.select` — alias of `listAll` per SPEC §12.9
  (selection is client-side; the server just exposes the list).
- `nuwiki.wiki.openIndex` — args `{ wiki? }` accepting an index id
  (number), name (string), or numeric string. Returns
  `{ uri, name, tab: false }` for `<root>/index<file_extension>`.
- `nuwiki.wiki.tabOpenIndex` — same payload but `tab: true` so
  the client knows to open in a new tab/split.
- `nuwiki.wiki.gotoPage` — args `{ page, wiki? }`. Prefers an
  indexed URI when the page is already known; otherwise builds
  `<root>/<page><file_extension>` directly. Drives the
  `:VimwikiGoto {name}` compat command.

Tests: 12 new in `phase18_multi_wiki.rs` covering parser shapes for
`wiki<N>:` / `wn.<Name>:` (and that plain `[[Home]]` stays
`LinkKind::Wiki`), multi-wiki config loading + v1.0 single-root
desugaring, `WikiId` sequential assignment, longest-prefix URI
routing for nested roots, the cross-wiki resolution semantics
(target wiki's index wins when both wikis define the same page
name), missing-wiki fallback, the COMMANDS list completeness, and a
link-kind matrix covering all nine kinds. Total 369 tests pass;
fmt + clippy clean.

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

SPEC.md

@@ -1382,26 +1382,33 @@ Deferred because the value proposition over the existing
 `workspace/rename` flow is marginal and the editor-side support is
 inconsistent across clients.
 
-### 13.3 v1.1 phases 18–19
+### 13.3 v1.1 phase 19
 
 Tracked in §10 with status `⏳`. Not "missing" from earlier phases —
-they're future work in the v1.1 roadmap:
+it's future work in the v1.1 roadmap:
 
-- **Phase 18** — Multi-wiki. Data structures (Wiki aggregate,
-  per-wiki index) already landed in Phase 11; this phase is the
-  config-shape change and the picker commands.
 - **Phase 19** — Editor glue v2: `:Vimwiki*` compat layer, default
   keymaps, text objects, folding (P14 already resolved in favour of
   LSP `foldingRange` + `foldexpr` fallback).
 
-Phase 17 (HTML export commands) shipped with the focused command
-surface (`currentToHtml` / `allToHtml[Force]` / `browse` / `rss`),
-template-file + fallback resolution, the `{{date}}` / `{{root_path}}`
-/ `{{toc}}` / `{{css}}` template variables, and `did_save` auto-export
-when `auto_export = true`. `color_dic` integration with the
-`ColorNode` renderer is still open (it's only useful once `colorize`
-from §13.1 lands), as is the §13.1 `link.pasteUrl` follow-up that
-depends on the same export config.
+Earlier v1.1 phases now shipped:
+
+- Phase 17 (HTML export commands) — `currentToHtml`,
+  `allToHtml[Force]`, `browse`, `rss`, template-file + fallback
+  resolution, `{{date}}` / `{{root_path}}` / `{{toc}}` / `{{css}}`
+  template variables, and `did_save` auto-export when
+  `auto_export = true`. `color_dic` integration with the `ColorNode`
+  renderer is still open (only useful once §13.1 `colorize` lands),
+  as is the §13.1 `link.pasteUrl` follow-up that depends on the same
+  export config.
+- Phase 18 (multi-wiki) — `wikis = [...]` config shape was already
+  accepted by Phase 11 plumbing; this phase added the cross-wiki
+  interwiki resolver (`Backend::resolve_target_uri` /
+  `heading_range_for` route `[[wiki<N>:Page]]` and
+  `[[wn.<Name>:Page]]` to the destination wiki's index) and four
+  picker commands: `nuwiki.wiki.listAll`, `nuwiki.wiki.select`,
+  `nuwiki.wiki.openIndex`, `nuwiki.wiki.tabOpenIndex`, plus
+  `nuwiki.wiki.gotoPage` for the `:VimwikiGoto` compat surface.
 
 ### 13.4 §9 syntax checklist status