P6 plan, owner review folded in: caller travels with the job, 1–2 round-robin workers, busy retry, summary reads, local-only tests, 8 tasks

Kyle: "Fold them into the plan." What changed and why is in docs/P6.md §14. In short:
- FetchFunc gets the *auth.Caller (the given api test could not compile, and a fetch with no
  caller would bypass per-caller gates, the pool limit, the cache class and redaction);
- store reads never bump the lease (task 01's rule contradicted its own test); the API Touches;
- 1-2 workers round-robin over live jobs, so a batch holds at most 2 of the 8 global slots;
  busy targets retry in the runner (bounded); batch PDFs use their own pool name; 210 s deadline;
- the api tests no longer touch the network or race the cancel; private names via fake DNS;
- failed/cancelled jobs are not reused; GET is a summary plus GET …/{idx} and ?results=1;
- the 256 KiB body cap moves into ServeHTTP; a fixed metrics label for /v1/batch/{id};
- guard.PreCheck (given verbatim) replaces guard.ParseURL at submit;
- 5 tasks became 8 (refactor, wiring, endpoints and serve/config split), each restating the
  APIs it uses; every given test compiles against a panic-only skeleton and fails on it.

P6 starts after P5c: task 04 moves v1fetch, which P5b changes. Recheck the api given tests
against post-P5c master before the run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/p6/_files/dcache/batch_test.go

@@ -56,6 +56,20 @@ func TestBatchIDemLookup(t *testing.T) {
 	if _, ok := db.BatchJobByIDem("b", "k", 2000, time.Hour); ok {
 		t.Fatal("other caller must not match")
 	}
+	// Only a job that is live or has results is reused. A failed (e.g. killed by a restart)
+	// or cancelled job must not be handed back to a resubmit for the rest of its lease.
+	for _, c := range []struct {
+		status string
+		reuse  bool
+	}{{"queued", true}, {"done", true}, {"partial", true}, {"failed", false}, {"cancelled", false}} {
+		key := "k-" + c.status
+		if err := db.CreateBatchJob(BatchJob{ID: "j-" + c.status, Caller: "a", CallerClass: ClassFleet, IDemKey: key, Status: c.status, TargetsTotal: 1, CreatedAt: 100, Lease: 1000}); err != nil {
+			t.Fatal(err)
+		}
+		if _, ok := db.BatchJobByIDem("a", key, 2000, time.Hour); ok != c.reuse {
+			t.Errorf("status %s: reused=%v, want %v", c.status, ok, c.reuse)
+		}
+	}
 }
 
 func TestBatchLiveCounts(t *testing.T) {