feat(library): attribute a run to a mission, and prove what it contributed

`corpus_items.mission_id` has existed since the table landed and nothing
could populate it. `POST /api/library/runs` now accepts `missionId`, which
is the seam the wizard needs: a mission-driven run is the same run, tagged.

`corpus::contributed()` answers the question a continuous mission has to
be able to answer — did THIS run add anything new. Because `record` never
reassigns mission_id on conflict, the mission that first found a source
keeps the credit, so a rerun cannot inflate its own count by re-recording
what an earlier run already held. The test asserts exactly that: two
missions see the same paper, the finder reports 1 and the rerun reports 0.

This is the check the 0030-0044 generation of continuous research did not
have. It could run weekly forever and every run looked like success.

The test also earned its FK: the first version attributed to a bare UUID
and the database refused it. Attribution to a mission that does not exist
is not attribution, so the test now seeds real mission rows.

400 tests, clippy clean.

Co-Authored-By: Claude Opus 5 <[email protected]>

crates/cm-api/src/corpus.rs

@@ -291,6 +291,35 @@ pub async fn unseen(
         .collect())
 }
 
+/// How many NEW sources a mission contributed.
+///
+/// The verification predicate for a continuous research mission. `record`
+/// never reassigns `mission_id` on conflict, so the first mission to find a
+/// source keeps the credit and a rerun cannot inflate its own count by
+/// re-recording what an earlier run already had.
+///
+/// A mission whose answer is zero produced nothing, whatever its transcript
+/// says — which is the check the 0030-0044 generation of this feature lacked.
+pub async fn contributed(
+    pool: &sqlx::PgPool,
+    workspace_id: Uuid,
+    corpus_id: &str,
+    mission_id: Uuid,
+) -> Result<i64, String> {
+    let row: (i64,) = sqlx::query_as(
+        "SELECT count(*) FROM corpus_items
+          WHERE workspace_id = $1 AND corpus_id = $2 AND mission_id = $3
+            AND kind = 'source'",
+    )
+    .bind(workspace_id)
+    .bind(corpus_id)
+    .bind(mission_id)
+    .fetch_one(pool)
+    .await
+    .map_err(|e| format!("contributed({mission_id}): {e}"))?;
+    Ok(row.0)
+}
+
 /// Index every note in a checkout. Idempotent by construction.
 pub async fn index_vault(
     pool: &sqlx::PgPool,