feat(job-array-single-wal-record): add compact array push capnp variant

Adds Data union variant jobArray@6, reusing JobSubmit's existing
(jobid, arraySize, submit) shape verbatim -- no new struct, no
existing variant renumbered. Not yet wired into any push path
(tasks 5.1/5.2 own that).

Task 1.2 of job-array-single-wal-record.

src/include/typemap.h

@@ -133,6 +133,10 @@ typedef struct {
         xce_workers_data_t*  workers;
         xce_resource_data_t* resources;
         xce_resval_data_t*   resvals;
+        xce_job_submit_t*    job_array; /* job-array-single-wal-record 1.2: compact
+                                          * (base jobid, array size, one spec) push
+                                          * descriptor -- same JobSubmit shape as the
+                                          * client submit request and WAL payload. */
     } data;
 } xce_data_t;