Every time in UTC, and every day as YYYY-MM-DD (closes #313)

Every time the program stores, logs or compares is in UTC, whatever TZ says, and AbuseIPDB's day in `reputation.json` is a `YYYY-MM-DD` date. Times read from a state file are converted to UTC as they are read, so a file is written back with `Z` times only. A test runs `sww waf` with `TZ=Pacific/Kiritimati` and checks the process line, the request line and `clients.json`. `SPEC.md` states the rule and `AGENTS.md` makes it an iron rule.

- The rate limit's `day` and `day_bytes` buckets keep a start time: each is a 24-hour bucket begun at an instant, not a calendar day.
- New dependency `cloud.google.com/go`, of which only `civil` is built.
- No migration: a `reputation.json` holding the old day form fails to load, naming the file.

Model: opus-5-5

go.sum

@@ -1,3 +1,5 @@
+cloud.google.com/go v0.123.0 h1:2NAUJwPR47q+E35uaJeYoNhuNEM9kM8SjgRgdeOJUSE=
+cloud.google.com/go v0.123.0/go.mod h1:xBoMV08QcqUGuPW65Qfm1o9Y4zKZBpGS+7bImXLTAZU=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=