feat(job-array-one-based-index): classify every job submission first (task 2.1)

handle_jobsubmit classifies (first_jobid, array_size) before the field
checks, the before-event hook and any registry access: a tagged array
starting at index 0 is answered with the new E_XCE_ARRAY_START0 and an
ERROR log asking for the client to be upgraded (7.1, wins per 7.9);
size 0, tags 2/3, untagged multi-element submissions and arrays whose
last element exceeds 32767 (checked without addition) get -EINVAL.
Nothing is stored for a rejected submission. This also closes a crash:
a size-0 submission used to reach payload building and overflow calloc.
Existing array tests move to tagged block|1 first IDs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/include/xce/errnox.h

@@ -23,6 +23,8 @@ enum {
     E_XCE_REDIRECT,   /* Standby is redirecting client to primary           */
     E_XCE_NOT_READY,  /* Controller in CTRL_INIT or CTRL_ELECTING state     */
     E_XCE_STANDBY,    /* Controller in CTRL_STANDBY; op is primary-only     */
+    E_XCE_ARRAY_START0, /* Array submission numbered from 0; numbering starts
+                         * at 1, the submitting client must be upgraded    */
     E_XCE_MAX
 };