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.

.kiro/specs/job-array-single-wal-record/tasks.md

@@ -8,7 +8,7 @@
   - _Requirements: 1.1, 1.2, 1.3_
   - _Boundary: WAL Array Submission_
 
-- [ ] 1.2 (P) Add the compact array push wire variant
+- [x] 1.2 (P) Add the compact array push wire variant
   - Add one new additive variant to the existing controller-to-scheduler/apiserver push message, reusing the client submit request's existing (base jobid, array size, one spec) shape verbatim — no new struct
   - Observable: the schema compiles and generates encode/decode functions for the new variant without touching any existing variant's wire layout
   - _Requirements: 6.1, 6.2_