feat(scheduler-dispatch-commit): add dispatch-fail retry counter to job_t (task 1.3)

Add uint32_t dispatch_fail_count to the job_t wrapper struct (xce/types.h,
alongside submit_lsn, same in-memory-only/reset-on-failover precedent),
per Requirement 4.3. Data model only -- xce_job_t (wire type) untouched,
job.c untouched; increment/reset wiring and the bounded-retry threshold
check are tasks 3.1/3.3.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/include/xce/types.h

@@ -487,6 +487,15 @@ typedef struct {
      * serialized (wire type xce_job_t above is untouched). 0 = unknown /
      * pre-feature = always exportable (see job_submit_committed, cluster.h). */
     uint64_t submit_lsn;
+    /* scheduler-dispatch-commit (1.3): consecutive retryable dispatch-failure
+     * count for this job, with no intervening successful dispatch. In-memory
+     * only, following submit_lsn's exact precedent above — never serialized
+     * (wire type xce_job_t above is untouched), reset to 0 by a successful
+     * dispatch. Resetting to 0 after a controller failover is an acceptable,
+     * safe degradation (more retries allowed, never fewer, never a
+     * misrouted terminal/retry decision) — see Controller Dispatch-Failure
+     * Classification, design.md. */
+    uint32_t dispatch_fail_count;
 } job_t;
 
 typedef struct {