feat(app): SearchResponse.trace + opts.trace threading (fb-37)

Adds the `trace: Option<SearchTrace>` field to `SearchResponse` and
threads `SearchOpts.trace` through `App::search_with_opts`. When the
caller sets `opts.trace = true` the path bypasses the LRU search cache
and runs through `HybridRetriever::search_with_trace`, which dispatches
all 3 SearchModes internally; this means `--trace` requires embeddings
(same constraint as `--mode hybrid`). The non-trace path keeps its
exact prior behavior with `trace: None` stamped on the response.

Picked up Task 1 / Task 3 follow-ups in the same commit so the
workspace compiles: SearchOpts struct-literals in kebab-cli/main.rs +
kebab-mcp/tools/search.rs default the new `trace` field to false, and
the schema-wrapper test in kebab-cli/wire.rs fills the new
media_breakdown / lang_breakdown / index_bytes / stale_doc_count fields
on Stats with `Default::default()`.

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

crates/kebab-cli/src/main.rs

@@ -732,6 +732,7 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
                 max_tokens: *max_tokens,
                 snippet_chars: *snippet_chars,
                 cursor: cursor.clone(),
+                trace: false,
             };
             // p9-fb-34: budget-aware path. --no-cache still bypasses the
             // App-level LRU; wire wrapper applies regardless.

crates/kebab-cli/src/wire.rs

@@ -264,6 +264,7 @@ mod tests {
             hits: vec![],
             next_cursor: Some("opaque-cursor-abc".to_string()),
             truncated: true,
+            trace: None,
         };
         let v = wire_search_response(&r);
         assert_eq!(schema_of(&v), Some("search_response.v1"));
@@ -303,6 +304,10 @@ mod tests {
             stats: Stats {
                 doc_count: 1, chunk_count: 2, asset_count: 1,
                 last_ingest_at: None,
+                media_breakdown: Default::default(),
+                lang_breakdown: Default::default(),
+                index_bytes: Default::default(),
+                stale_doc_count: 0,
             },
         };
         let v = wire_schema(&schema);