feat(audit): every row keeps its date; dividers for Today, Yesterday, Last 7 days, Older (#5214)

The operator found "Today 08:03" in place of a date odd on the audit log,
and asked for the recency groups as dividers instead. |when now always
writes the date ("Thu 8 Oct 08:03"); mark_day_groups gives each entry its
group in the reader's zone, and a divider row opens each group. A scrolled
page carries the group the last one ended in, so it is not opened twice.

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

steward/core/formatting.py

@@ -9,7 +9,7 @@ here. A moment is written in the reader's own zone, on a 24-hour clock
 """
 from __future__ import annotations
 
-from datetime import datetime, timedelta, timezone, tzinfo
+from datetime import datetime, timezone, tzinfo
 from functools import lru_cache
 from typing import Any
 from zoneinfo import ZoneInfo, ZoneInfoNotFoundError, available_timezones
@@ -120,29 +120,40 @@ def ago(when: datetime | None, now: datetime | None = None, tz: tzinfo | None =
 
 
 def when(moment: datetime | None, tz: tzinfo | None = None, now: datetime | None = None) -> Markup:
-    """The exact moment in the reader's zone on a 24-hour clock, as short as
-    it can be read: 'Today 06:58', 'Yesterday 23:41', 'Tomorrow 02:00',
-    'Tue 6 Oct 14:12' this year, '6 Oct 2025 14:12' before it."""
+    """The exact moment in the reader's zone on a 24-hour clock, always with
+    its date: 'Thu 8 Oct 06:58' this year, '6 Oct 2025 14:12' before it. A
+    list that groups by recency says Today or Yesterday in a divider
+    (day_group), never in place of the date (#5214)."""
     if moment is None:
         return Markup(NONE)
     tz = tz or timezone.utc
     moment = _utc(moment)
     local = moment.astimezone(tz)
-    today = (now or datetime.now(timezone.utc)).astimezone(tz).date()
+    this_year = (now or datetime.now(timezone.utc)).astimezone(tz).year
     day, hm = local.date(), f"{local:%H:%M}"
-    if day == today:
-        text = f"Today {hm}"
-    elif day == today - timedelta(days=1):
-        text = f"Yesterday {hm}"
-    elif day == today + timedelta(days=1):
-        text = f"Tomorrow {hm}"
-    elif day.year == today.year:
+    if day.year == this_year:
         text = f"{local:%a} {day.day} {local:%b} {hm}"
     else:
         text = f"{day.day} {local:%b} {day.year} {hm}"
     return _time(moment, local, text)
 
 
+DAY_GROUPS = ("Today", "Yesterday", "Last 7 days", "Older")
+
+
+def day_group(moment: datetime, tz: tzinfo | None = None, now: datetime | None = None) -> str:
+    """Which of DAY_GROUPS a past moment falls in, by calendar day in the
+    reader's zone: the dividers of a newest-first log."""
+    tz = tz or timezone.utc
+    day = _utc(moment).astimezone(tz).date()
+    back = ((now or datetime.now(timezone.utc)).astimezone(tz).date() - day).days
+    if back <= 0:
+        return DAY_GROUPS[0]
+    if back == 1:
+        return DAY_GROUPS[1]
+    return DAY_GROUPS[2] if back < 7 else DAY_GROUPS[3]
+
+
 def status_class(value: Any, kind: str, thresholds: dict) -> str:
     """Grade a reading against Settings → Thresholds (to_thresholds_cfg):
     'ok', 'warn' or 'crit', or 'unknown' when there is no reading.