Stop one subject taking every notification slot

Connor noticed 5 of 5 alerts were Middle East. Measuring showed that was not
proportionate: Middle East blocks are ~11% of the eligible corpus while
software-systems alone is 25%, yet Middle East took 35% of the top 20 and
software took none.

Two mechanisms, both measured:

1. Five conflict-* blocks could each claim their own alert slot. Interest blocks
   now carry a `group`, and routing enforces max_alerts_per_group_per_day, so
   related blocks share one quota. This alone moved the mix from 5/5 Middle East
   to three distinct groups.

2. Z-scoring subtracts each block's MEAN, which silently encodes how much of the
   corpus that block matches. software-systems has the HIGHEST mean raw
   similarity of any block (0.537) and the LOWEST max z (2.51);
   conflict-mideast-israel has a lower mean (0.460) and reaches 3.47. A topic is
   penalised for being well covered in your own feed list.

Added `normalize = "top-anchored"` as an alternative that rescales each block by
its own upper tail instead of its mean, leaving `weight` to express preference
rather than corpus composition.

Also ruled out a plausible-sounding cause rather than assuming it: HN and
Lobsters items are title-only and much shorter than news articles, so terse
titles seemed likely to match prose probes poorly. Measured the opposite --
title-only HN items score 0.618 against the software block, ABOVE Phoronix/LWN
with summaries (0.610) and Ars Technica (0.540). Short text is not the problem;
prevalence is.

profile/config.toml

@@ -72,6 +72,9 @@ corroboration_depth_weight = 0.2
 # Daily budgets. Deliberately small: the point of the system is a handful of
 # well-chosen interruptions, not a feed.
 interest_per_day = 3
+# Max alerts per quota group per day (see `group` in profile.toml). Stops one
+# subject taking every slot.
+max_alerts_per_group_per_day = 1
 important_per_day = 2
 # Hard cap on waking the reader during quiet hours, inside important_per_day.
 important_per_night = 1