phase 11: plumbing prereqs — Wiki, edits, config, snapshot, diags

Cross-cutting infrastructure that every v1.1 phase depends on. Lands as
a numbered phase before any user-visible feature so Phases 12–19 stay
mechanical.

5 pieces:

- `config.rs` — `Config` + `WikiConfig` + `DiagnosticConfig` + `LinkSeverity`.
  Serde-deserialised from `initialization_options`. v1.0 single-wiki
  shape (`wiki_root` + `file_extension` + `syntax`) desugars to one-entry
  `wikis = [...]` per P16. `~/...` expansion without pulling in `dirs`.
  Fallbacks: workspace_folders[0] → deprecated root_uri → empty list.

- `wiki.rs` — `Wiki` aggregate (id + WikiConfig + Arc<RwLock<Index>>).
  `build_wikis(&[WikiConfig])` materialises the list; `resolve_uri_to_wiki`
  picks the longest-prefix root match so nested wikis route correctly.
  Single-entry today, multi-entry once Phase 18 lands.

- `edits.rs` — `WorkspaceEditBuilder` + `text_edit_replace/insert/delete`
  + `op_rename/delete/create`. Handles the UTF-8/UTF-16 conversion at the
  span boundary so each Phase 13+ command stays a 3-liner. Builder picks
  `documentChanges` when file ops are present (LSP requires it), `changes`
  map otherwise (works for LSP 3.15- clients). File ops precede content
  edits in the output so created/renamed files exist when edits apply.
  `DeleteFile` is built without `annotation_id` — lsp-types 0.94.x ships
  it that way; later versions added the field.

- `Backend` refactor — `index: Arc<RwLock<WorkspaceIndex>>` replaced by
  `wikis: Arc<RwLock<Vec<Wiki>>>` + `config: Arc<RwLock<Config>>`. New
  `wiki(id)` / `default_wiki()` / `wiki_for_uri(uri)` helpers. Every v1.0
  handler (definition, references, hover, completion, workspace/symbol,
  update_document) goes through `wiki_for_uri`. `initialize` builds wikis
  from `Config::from_init_params`; `initialized` spawns one indexer task
  per wiki with the existing `window/workDoneProgress` wrapper. New
  `did_change_configuration` handler re-applies settings (rebuild
  semantics revisited in Phase 18). The old `workspace_root_from_params`
  helper is gone.

- `read_or_open` + `DocSnapshot` (scaffolded, `#[allow(dead_code)]` until
  Phase 13's `workspace/rename` consumes it). Returns the live document
  when held in the documents map, otherwise async-reads from disk and
  re-parses so cross-document operations get accurate text and spans
  instead of trusting stale `WorkspaceIndex` data.

- `collect_diagnostics` composable collector. Same `ErrorNode` walk
  today; takes `Option<&WorkspaceIndex>` + `page_name` + `&DiagnosticConfig`
  so Phase 15 can drop a link-health source in without further refactor.
  `ast_diagnostics` kept as a thin wrapper for the v1.0 test surface.

New deps: `serde` + `serde_json` (already transitive through tower-lsp;
elevated to direct deps for clarity and stability). `tokio` gains the
`fs` feature for `read_or_open`'s async disk read.

Tests (19 new in `phase11_plumbing.rs`):
- Edits: UTF-8 passthrough + UTF-16 conversion (`héllo`), insert/delete
  shape, builder mode selection, file-op accumulation, `op_create` round-trip
- Config: defaults, v1.0 desugar, explicit list overrides legacy, empty
  JSON, invalid JSON preserves prior state
- Wiki: sequential id assignment, longest-prefix resolution, no-match,
  `Wiki::contains`
- Diagnostics: error-node composition, link-health stub returns empty in
  Phase 11

All 172 v1.0 tests still pass — backward compatible.

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

crates/nuwiki-lsp/src/wiki.rs

@@ -0,0 +1,70 @@
+//! `Wiki` aggregate — per-wiki state, even when there's only one.
+//!
+//! v1.0 shipped with a single `Arc<RwLock<WorkspaceIndex>>` on `Backend`.
+//! Phase 11 lifts that into a `Wiki` aggregate so Phase 18 (multi-wiki)
+//! only changes config shape, not data flow. Phases 12–17 always operate
+//! on a `Wiki`; in practice the `wikis` `Vec` has one entry until 18.
+
+use std::path::Path;
+use std::sync::{Arc, RwLock};
+
+use tower_lsp::lsp_types::Url;
+
+use crate::config::WikiConfig;
+use crate::index::WorkspaceIndex;
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash, Default)]
+pub struct WikiId(pub u32);
+
+#[derive(Clone)]
+pub struct Wiki {
+    pub id: WikiId,
+    pub config: WikiConfig,
+    pub index: Arc<RwLock<WorkspaceIndex>>,
+}
+
+impl Wiki {
+    pub fn new(id: WikiId, config: WikiConfig) -> Self {
+        let index = Arc::new(RwLock::new(WorkspaceIndex::new(Some(config.root.clone()))));
+        Self { id, config, index }
+    }
+
+    /// True when `uri` resolves to a file under this wiki's root.
+    pub fn contains(&self, uri: &Url) -> bool {
+        let Ok(path) = uri.to_file_path() else {
+            return false;
+        };
+        path.starts_with(&self.config.root)
+    }
+
+    /// Length of the wiki's root path (component count) — used to break
+    /// ties in nested-root layouts.
+    pub fn root_depth(&self) -> usize {
+        self.config.root.components().count()
+    }
+
+    pub fn root(&self) -> &Path {
+        &self.config.root
+    }
+}
+
+/// Pick the wiki whose root is the longest prefix of `uri`. Returns the
+/// wiki id, not a guard, so callers can release the read lock before
+/// awaiting.
+pub fn resolve_uri_to_wiki(wikis: &[Wiki], uri: &Url) -> Option<WikiId> {
+    wikis
+        .iter()
+        .filter(|w| w.contains(uri))
+        .max_by_key(|w| w.root_depth())
+        .map(|w| w.id)
+}
+
+/// Build the initial `Vec<Wiki>` from a list of `WikiConfig`s. Always
+/// assigns ids by position so `WikiId(0)` is the default wiki.
+pub fn build_wikis(configs: &[WikiConfig]) -> Vec<Wiki> {
+    configs
+        .iter()
+        .enumerate()
+        .map(|(i, cfg)| Wiki::new(WikiId(i as u32), cfg.clone()))
+        .collect()
+}