v3.22.0 (review fixes): the ownership reconciliation and the import guards

The suite/script ownership invariant could break silently: a suites import
with mode "replace" wiped the suites while their scripts stayed behind
(suite_id pointing at gone suites - invisible in the UI), a scripts import
with mode "replace" left every suite's member list dangling, and a script
whose suite_id disagreed with its listing suite stayed inconsistent.

- server: reconcile_scripts_to_suites() repairs the invariant - a script a
  suite lists adopts that suite (the member list is authoritative), a script
  no suite lists joins "Imported scripts"; migrate_scripts_to_suites()
  delegates to it (the startup migration is the same pass) and the suites
  import runs it after loading, so a "replace" adopts the wiped suites'
  scripts instead of losing them.
- the scripts import refuses mode "replace" while any suite still lists
  scripts (400 naming the dangling references; merge is the normal path).
- nits from the review: a legacy numeric `event` is no longer parsed as hex
  in the step summary/error check (0x0A displayed as 0x10), and an integral
  float memory quota (a JSON client's 200.0) is accepted.
- tests: the reconciliation (missing suite, listed-but-mismatched,
  idempotent), the scripts-replace guard (refused/allowed), the
  suites-replace adoption, the float quota, the numeric-event summary.
  Tests: 838 frontend + 800 Python; sw cache simple-v402.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v401';
+const CACHE = 'simple-v402';
 const URLS = [
   'index.html',
   'help.html',