apply go fix modernizations across the workspace

`go fix` carries the modernize analyzers now, and the tree had drifted behind
them. This is the mechanical result, reviewed rather than trusted: the tool is
capable of rewriting code into something that no longer tests or does what it
did, so every non-test change was read individually and the concurrency-bearing
packages were re-run under -race.

Production code, four changes, all semantics-preserving:

  - leases/manager.go: wg.Add(1) + go + defer wg.Done() becomes wg.Go. The
    comment above that function turns on Add happening before the goroutine
    starts, so that a Wait cannot return before the worker has run. wg.Go does
    the Add synchronously on the calling goroutine, so the invariant it
    describes still holds.
  - auth/token/handler.go: strings.Fields -> strings.FieldsSeq, same splitting,
    iterated rather than allocated.
  - hold/gc/gc.go: a hand-written map copy -> maps.Copy.
  - hold/pds/scan_broadcaster.go: three-clause loop -> range over int.

The rest are tests. The one worth naming is carstore_contention_test.go, where a
careless rewrite could have quietly stopped exercising contention: go fix
converted the reader and side-table goroutines to loopWG.Go but correctly
declined to touch the writer loop, which passes its index as a parameter. The
writer/reader/side-table shape and the stop channel are unchanged, so the test
still contends over the same carstore transactions.

Verified: go build for hold and appview, `make lint` 0 issues, the deploy and
credential-helper modules 0 issues, `make test` green across all 43 packages,
and -race green on leases, hold/pds, hold/gc and auth/token. The scanner module's
two lint findings are unchanged from HEAD and are in files go fix never touched.

Kept separate from the HTTP/2 commit so that one stays readable, and so this can
be reverted on its own if a modernization turns out to matter.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TA9D4DjaLZTvzQ7dJbu4eg

pkg/auth/token/handler.go

@@ -391,7 +391,7 @@ func collectScopes(values []string) []string {
 	var scopes []string
 	seen := make(map[string]bool)
 	for _, value := range values {
-		for _, scope := range strings.Fields(value) {
+		for scope := range strings.FieldsSeq(value) {
 			if seen[scope] {
 				continue
 			}