fix(fb-34): align next_cursor semantics with docs (PR #125 round 2)

Previous round-1 fix dropped the speculative cursor branch on
the truncated path, leaving a contradiction with the docs:
- snippet-only shrunk → cursor emitted (returned == k_effective)
- k-popped → cursor null (returned < k_effective)
But docs promised the opposite.

R2 resolution: emit cursor whenever more hits may be reachable
(either retriever filled the page OR budget popped hits — the
popped ones remain fetchable from offset+returned). Drop the
artificial "widen vs paginate" copy; truncated and next_cursor
are now independent signals — caller may do either or both.

Updates: app.rs::search_with_opts logic + SearchResponse doc +
schema description + SKILL.md two bullets + max_tokens=0 test
asserts cursor IS emitted on k-pop case.

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

docs/wire-schema/v1/search_response.schema.json

@@ -9,6 +9,6 @@
     "schema_version": { "const": "search_response.v1" },
     "hits":           { "type": "array", "description": "search_hit.v1[]" },
     "next_cursor":    { "type": ["string", "null"], "description": "Opaque base64 cursor for next page; null when no more hits." },
-    "truncated":      { "type": "boolean", "description": "True when budget forced snippet shortening or k reduction. When `next_cursor` is also non-null, k was reduced — paginate via cursor. When `next_cursor` is null but `truncated: true`, only snippets were shortened — widen `max_tokens` to get fuller snippets." }
+    "truncated":      { "type": "boolean", "description": "True when budget forced snippet shortening or k reduction. Independent of `next_cursor`: caller may widen `max_tokens` (re-issue same query) or follow `next_cursor` (advance through more hits) or both." }
   }
 }