fix(frontend): peek and restart take <array-id>[k] and written job IDs

- apiserver peek (parseJobIDParam): <array-id>[k] now resolves to the
  element; it used to drop [k] and look up the array identity itself
  (and panicked in parseArrayName on some inputs). A job number still
  names that job exactly (an element by its own XCE_JOBID); ranges and
  invalid selections are rejected.
- apiserver restart: the decimal-only pre-check (isValidJobId, now
  unused and removed) rejected the written form c-a-ts-seq[k];
  ParseIdentitySelection, already used for the lookup, validates it.
- xcecli jobs restart converts its argument with ToAPIJobID like the
  control commands, so written IDs work and invalid ones fail locally.
- docs: the peek/restart caveat in the Job arrays section.

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

src/frontend/apiserver/controllers/output.go

@@ -9,7 +9,6 @@ import (
 	"apiserver/common"
 	"apiserver/models"
 	"apiserver/store"
-	"apiserver/utils"
 
 	logger "apiserver/utils"
 
@@ -220,25 +219,29 @@ func resolveOutputPath(path string, job models.JobInfo) string {
 	return path
 }
 
-// parseJobIDParam parses a job ID string and returns the corresponding JobInfo.
+// parseJobIDParam parses the job peek names and returns the corresponding
+// JobInfo: a job number (an ordinary job, or an element by its own number,
+// XCE_JOBID) names that job exactly; <array-id>[k] names element k. A range
+// or an invalid selection is an error -- peek shows one job.
 func parseJobIDParam(jobidStr string) (models.JobInfo, error) {
-	// Use the same parsing logic as GetJobByIDs
-	jobArrInfo, err := parseArrayName(jobidStr)
-	if err != nil {
-		return models.JobInfo{}, err
+	raw := strings.TrimSpace(jobidStr)
+	if n, err := strconv.ParseUint(raw, 10, 64); err == nil {
+		return models.JobInfo{Unique_JobID: int64(n), ArrayIdx: backend.ExtractArrayIdx(n)}, nil
 	}
 
-	if !utils.IsNumeric(jobArrInfo.KeyID) {
-		return models.JobInfo{}, fmt.Errorf("bad jobid: %s", jobArrInfo.KeyID)
+	sel, selErr := ParseIdentitySelection(raw)
+	if selErr != nil {
+		return models.JobInfo{}, selErr
 	}
-
-	uniqueJobID, err := strconv.ParseInt(jobArrInfo.KeyID, 10, 64)
-	if err != nil {
-		return models.JobInfo{}, fmt.Errorf("bad jobid: %s", jobArrInfo.KeyID)
+	switch sel.Form {
+	case SelSingle:
+		return models.JobInfo{
+			Unique_JobID: int64(backend.MakeElementID(sel.Key, sel.Lo)),
+			ArrayIdx:     sel.Lo,
+		}, nil
+	case SelRange:
+		return models.JobInfo{}, fmt.Errorf("'%s' names several jobs; peek one element as <array-id>[k]", raw)
+	default:
+		return models.JobInfo{Unique_JobID: int64(sel.Key)}, nil
 	}
-
-	return models.JobInfo{
-		Unique_JobID: uniqueJobID,
-		ArrayIdx:     0, // peek defaults to base job; array idx can be added later
-	}, nil
 }