fix(search): move scope→item-type taxonomy into Rust (UR-049, DR-063)

Stage 1 of scoped-search-boundary-implementation.md — the query side.

scoped-search-boundary.md diagnosed this leak, specified the fix in
detail, and became the justification for the boundary rule in CLAUDE.md,
the check:boundary tripwire, and the spec-review checklist. The fix was
never built: SCOPE_ITEM_TYPES was still live in searchScope.ts, called by
library.ts, and no SearchScope existed anywhere in src-tauri/. The rule's
own founding violation was still shipping.

Rust now owns the taxonomy:

  pub enum SearchScope { All, Music, Movies, Tv }
  impl SearchScope { pub fn item_types(self) -> Option<Vec<String>> }

- SearchOptions gains `scope`, resolved by resolve_scope(). Scope wins
  over include_item_types, which stays for the non-search get_items
  callers that legitimately request one concrete type.
- repository_search resolves the scope ONCE, before the cache/server
  paths diverge, so online and offline filter identically — the failure
  mode most likely to go unnoticed.
- All expands to None (no filter), not the union of the other scopes:
  an explicit includeItemTypes list would silently drop People, folders,
  and any type nobody enumerated.
- searchScope.ts re-exports SearchScope from generated bindings instead
  of a hand-written union, and no longer names an item type for search.
- library.ts sends { scope }.

8 Rust tests written first, confirmed failing on "use of undeclared type
SearchScope" before the implementation existed.

The frontend tests that asserted includeItemTypes contents were rewritten
to assert the opaque scope is sent and includeItemTypes is absent —
keeping the old assertions would require the frontend to know the
taxonomy again, defeating the fix. The expansion is now asserted in Rust.

Verified the spec's headline criterion by hashing every src/ file, adding
"AudioBook" to the Music scope in Rust, and re-hashing: zero frontend
files change. That criterion failed before this commit.

Stage 2 (result-side grouping: GROUP_ITEM_TYPES, GroupedSearchResult on
both search payloads) remains open.

src/lib/stores/library.ts

@@ -5,7 +5,7 @@ import { writable, derived } from "svelte/store";
 import { listen, type UnlistenFn } from "@tauri-apps/api/event";
 import type { Library, MediaItem, SearchResult, Genre } from "$lib/api/types";
 import type { SearchOptions } from "$lib/api/bindings";
-import { scopeItemTypes, type SearchScope } from "$lib/utils/searchScope";
+import type { SearchScope } from "$lib/utils/searchScope";
 import { auth } from "./auth";
 
 /**
@@ -227,12 +227,13 @@ function createLibraryStore() {
   /**
    * Search the library, optionally narrowed to a scope.
    *
-   * `scope` is additive and defaults to `all`, which sends no
-   * `includeItemTypes` at all — see scopeItemTypes() for why that differs from
-   * listing every type. Both the online and offline repository paths already
-   * honour the filter.
+   * The scope is sent **opaque**; Rust expands it into Jellyfin item types
+   * (`SearchScope::item_types()`) before the cache and server paths diverge, so
+   * online and offline results filter identically. `all` resolves to no filter
+   * at all — not the union of the other scopes, which would drop People and
+   * folders.
    *
-   * TRACES: UR-049 | DR-065
+   * TRACES: UR-049 | DR-063, DR-065
    */
   async function search(query: string, scope: SearchScope = "all") {
     // Bump the request id for every call (including clears) so any in-flight
@@ -259,10 +260,9 @@ function createLibraryStore() {
       // Phase 1: the command resolves with instant local-cache results. The
       // merged (cache + server) union arrives later via the `search-event`
       // listener above, tagged with this same requestId.
-      const itemTypes = scopeItemTypes(scope);
-      const options: SearchOptions = { limit: 10000 };
-      // Omit the key entirely for the `all` scope rather than sending null.
-      if (itemTypes) options.includeItemTypes = itemTypes;
+      // Send the opaque scope; Rust expands it to item types. The frontend
+      // never names a Jellyfin item type in connection with search.
+      const options: SearchOptions = { limit: 10000, scope };
 
       const result = await Promise.race([
         repo.search(query, options, requestId),