P6 review: runner shutdown, cancel-while-busy, concurrent submits, batch-off 503, transactional store, CLI 202

Owner review of p6 (Ornith tasks 01-09), with an independent reviewer pass whose findings I
confirmed against the code. Every fix has a regression test that fails on the old code.

internal/batch
- Close let the workers keep draining every live job under the cancelled context, failing
  every target; they now stop at once, store nothing on a failure during shutdown, and leave
  the job live for MarkInFlightFailed("restart").
- A job cancelled while its in-flight target came back busy stayed in the rotation forever
  (depth gauge stuck, job held in memory); it now leaves. The task 03 design had this gap: mine.
- Submit's idempotency lookup, caps and create ran unlocked: 20 concurrent submits by one
  caller made 8 jobs. Serialised with submitMu.
- A failed target insert left a queued job no worker would run (holding the caller's cap);
  it is marked failed.
- A second concurrent cancel returned true and counted twice.
- Config: zero OR negative values take the defaults (negative Workers panicked the
  WaitGroup; a negative TTL or byte budget would sweep or evict everything).
- Retry timing used the pre-fetch time (a 5 s busy answer made RetryWait moot).
- The rotation cursor is kept within [0, len(live)] (it was absolute and drifted on removal).
- The unused in-memory callers map is gone (Submit racing Close wrote to a nil map).
internal/dcache
- PutBatchTargets, SweepBatch and eviction are transactional; the sweep never removes a
  queued or running job.
internal/api
- Every /v1/batch endpoint answers 503 when batch is off; POST panicked on the nil runner.
- GET /{id}/{idx} bounds idx by the rows actually returned.
- batchFetch copies a *fetch.Error instead of mutating one the PDF pool may share.
internal/config
- ParseSize no longer overflows to a negative size.
cmd/sift
- sift batch treated 202 Accepted (submit) as an error and exited 1 with no message.

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

internal/config/batch_review_test.go

@@ -0,0 +1,10 @@
+package config
+
+import "testing"
+
+// P6 owner review: a size too large for int64 falls back to the default instead of wrapping negative.
+func TestParseSizeOverflow(t *testing.T) {
+	if got := ParseSize("9223372036854775807GiB", 7); got != 7 {
+		t.Fatalf("ParseSize(huge) = %d, want the default", got)
+	}
+}

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"math"
 	"os"
 	"strconv"
 	"strings"
@@ -145,7 +146,7 @@ func ParseSize(s string, def int64) int64 {
 		mult = 1
 	}
 	n, err := strconv.ParseInt(s, 10, 64)
-	if err != nil || n <= 0 {
+	if err != nil || n <= 0 || n > math.MaxInt64/mult { // the last: n*mult would overflow
 		return def
 	}
 	return n * mult