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/profile.toml

@@ -10,6 +10,10 @@ schema_version = 2
 
 # =========================================================== interests =======
 
+# `group` shares one notification quota across related blocks. Without it the
+# five conflict-* blocks could each claim an alert slot and fill a day with the
+# same subject -- which is what happened: 5 of 5 alerts were Middle East.
+#
 # --- foreign affairs, split from armed conflict ------------------------------
 # Connor: "my actual war interest is low, but unfortunately all wars are
 # important." The click history looked war-heavy only because world-news feeds
@@ -30,7 +34,8 @@ probes = [
 # The A-E ordering Connor gave, encoded as weights. Even the lowest band still
 # reaches him -- but through importance, on major developments only.
 [[interest]]
-id = "conflict-anti-imperial"          # (A) top of his ordering
+id = "conflict-anti-imperial"
+group = "conflict"          # (A) top of his ordering
 weight = 0.9
 probes = [
   "An armed liberation movement fights a colonial or occupying power for self-determination.",
@@ -39,7 +44,8 @@ probes = [
 ]
 
 [[interest]]
-id = "conflict-us-involved"            # (B)
+id = "conflict-us-involved"
+group = "conflict"            # (B)
 weight = 0.85
 probes = [
   "US forces carry out strikes and the administration defends the legal basis for the war.",
@@ -47,7 +53,8 @@ probes = [
 ]
 
 [[interest]]
-id = "conflict-mideast-israel"         # (C)
+id = "conflict-mideast-israel"
+group = "conflict"         # (C)
 weight = 0.8
 probes = [
   "Israeli forces strike targets in Gaza, Lebanon or Syria amid continuing escalation.",
@@ -56,7 +63,8 @@ probes = [
 ]
 
 [[interest]]
-id = "conflict-undercovered"           # (D)
+id = "conflict-undercovered"
+group = "conflict"           # (D)
 weight = 0.75
 probes = [
   "Fighting intensifies in Sudan as famine spreads and aid convoys are blocked.",
@@ -65,7 +73,8 @@ probes = [
 ]
 
 [[interest]]
-id = "conflict-general"                # (E) -- low interest, still important
+id = "conflict-general"
+group = "conflict"                # (E) -- low interest, still important
 weight = 0.45
 probes = [
   "Front-line fighting continues as both sides exchange drone and artillery strikes.",
@@ -137,6 +146,7 @@ probes = [
 
 [[interest]]
 id = "labor-inequality"
+group = "left-politics"
 weight = 0.85
 probes = [
   "Workers vote to unionize after a long organizing campaign; the employer contests the result.",
@@ -148,6 +158,7 @@ probes = [
 
 [[interest]]
 id = "left-movements"
+group = "left-politics"
 weight = 0.85
 probes = [
   "A socialist party wins a decisive election, shifting the governing coalition leftward.",
@@ -157,6 +168,7 @@ probes = [
 
 [[interest]]
 id = "anticolonial-sovereignty"
+group = "left-politics"
 weight = 0.8
 probes = [
   "Indigenous communities win recognition of land rights against extractive industry.",
@@ -176,6 +188,7 @@ probes = [
 
 [[interest]]
 id = "political-theory"
+group = "left-politics"
 weight = 0.7
 probes = [
   "An essay argues about socialism, imperialism and political economy, drawing on history rather than reporting a news event.",
@@ -204,6 +217,7 @@ probes = [
 
 [[interest]]
 id = "cincinnati"
+group = "local"
 weight = 0.9
 local = true
 probes = [
@@ -216,6 +230,7 @@ probes = [
 
 [[interest]]
 id = "kentucky-louisville"
+group = "local"
 weight = 0.6
 local = true
 probes = [
@@ -226,6 +241,7 @@ probes = [
 # coverage alongside Cincinnati city government.
 [[interest]]
 id = "ohio-state-politics"
+group = "local"
 weight = 0.75
 local = true
 probes = [