Normalise per group, and make the notification quota subject-based

Following Connor's point that corpus prevalence reflects how much an outlet
publishes rather than what deserves a notification: allocation should track
stated weights, not corpus composition in EITHER direction. Penalising a topic
for being prevalent is a bug; rewarding it would just amplify whoever publishes
most. This rules out the "reserve slots proportional to corpus share" model that
the previous commit's findings were pointing toward.

Two fixes:

1. Normalisation moves from per-block to per-GROUP. Taking the max across blocks
   gives a subject one draw per block, so foreign affairs -- six of 23 blocks --
   beat everything else by having more chances. It held 70% of the digest while
   software-systems, a quarter of the corpus, held none. Collapsing to the group
   before normalising prices that in. Digest conflict share 13 -> 9, space 2 ->
   4, and us-national appears at all.

2. The per-group quota now keys on the INTEREST group for both lanes. Importance
   blocks describe a kind of importance, not a subject, so two stories on one
   subject could each take a slot via different importance blocks -- which is
   what happened: both importance alerts were Middle East, one via
   war-escalation and one via economic-break.

Interest alerts went from five-of-five Middle East to Ohio redistricting,
political theory and tech policy.

Also demoted prerelease/nightly build entries. The first alert software-systems
ever won was "Nvim development (prerelease) build", which matches the block
correctly and is worthless as a notification.

Documented that switching normalizer requires re-tuning [routing] thresholds:
they are in z-score units, and leaving them at 2.0 under "top-anchored"
silences the interest lane entirely.

profile/profile.toml

@@ -369,6 +369,17 @@ probes = [
   "This week in review: the stories we covered, collected.",
 ]
 
+# Release feeds publish nightly and prerelease builds continuously. They match
+# software-systems correctly and are worthless as notifications -- the first
+# alert this block ever won was "Nvim development (prerelease) build".
+[[demote]]
+id = "prerelease-build"
+penalty = 1.5
+probes = [
+  "Nightly development prerelease build, automatically generated from the main branch.",
+  "Release candidate rc1: pre-release for testing, not recommended for production.",
+]
+
 # ============================================= the importance lane ===========
 # Independent of interest. Vetoes still apply here -- v0.2's bypass lane ignored
 # relevance entirely (gate.rs:302), which let blocklisted items fire at priority 5.