fix(#1927): stop plotting the usefulness composite on the traffic axis (#1960)

Closes #1927.

The scatter's **"Traffic share"** axis and the **"Traffic"** table
column both fell back to `usefulness_score` when `traffic_share_score`
was absent, with nothing marking the substitution. Two points on one
axis could therefore measure different things.

## They really are different metrics

Checked rather than assumed, in `cmd/server/usefulness_composite.go`:

```go
node["traffic_share_score"] = trafficRaw   // :147  the single traffic axis
node["usefulness_score"]    = composite    // :152  0.30*bridge + 0.25*coverage + ...
```

`openapi.go:178,182` describes them the same way. So the fallback put a
**composite** under a column and an axis that both promise share of
non-advert traffic.

Worth flagging, because it is easy to conclude the opposite: **#1456,
which introduced the fallback, was a rename of the display label** from
"Usefulness" to "Traffic share". That makes the two field names look
interchangeable, and I nearly stopped there. They are not: #672 later
gave `usefulness_score` its own composite meaning.

## Fix

Your first preference in the issue: drop the fallback rather than mark
it or relabel the axis.

A node with no `traffic_share_score` now reads as unknown, so the table
shows an em dash and the point is dropped from the plot by the existing
`plottable` filter (`analytics.js:2728`), exactly the way a node with no
bridge score already is. **No other change was needed** for that, which
is what makes this the cheap option of the three.

## Tests

The unit test that pinned the old behaviour is updated rather than
deleted, so the expectation is now recorded the right way round:

```js
assert(mapped[1].traffic === null && mapped[1].fav === false,
  'a node with only usefulness_score has no traffic value; it must not be substituted');
```

`test-repeater-metric-scatter.js`: 31 passed, 0 failed.

Also corrected a comment above `_toScatterPoints` that still documented
the removed fallback chain.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

public/analytics.js

@@ -2487,8 +2487,13 @@
         return age < th.degradedMs ? 'active' : age < th.silentMs ? 'degraded' : 'silent';
       }
       const pct = v => (v != null ? (v * 100).toFixed(1) + '%' : '—');
-      // #1456: prefer traffic_share_score, fall back to usefulness_score.
-      const trafficOf = n => (n.traffic_share_score != null ? n.traffic_share_score : (n.usefulness_score != null ? n.usefulness_score : null));
+      // #1927: no fallback to usefulness_score. They are different metrics:
+      // traffic_share_score is the single traffic axis, usefulness_score is the
+      // #672 composite (0.30*bridge + 0.25*coverage + ...), set separately at
+      // cmd/server/usefulness_composite.go:147 and :152. Substituting one for the
+      // other put a composite under a column and an axis that both promise share
+      // of non-advert traffic. A node without the metric now reads as unknown.
+      const trafficOf = n => (n.traffic_share_score != null ? n.traffic_share_score : null);
       function roleBadge(role) {
         // Route the unknown-role fallback through ROLE_COLORS.unknown (backed by
         // --mc-role-unknown / the shared Wong palette) instead of a hard-coded
@@ -2682,9 +2687,10 @@
   }
 
   // Map /api/nodes rows to plottable points. Pure and factored out of
-  // renderRepeaterMetricsTab so the repeater/room filter and the fallback
-  // chains (traffic_share_score → usefulness_score → null; name → pubkey
-  // prefix → '?') are unit-testable (#1760 review).
+  // renderRepeaterMetricsTab so the repeater/room filter and the name fallback
+  // (name → pubkey prefix → '?') are unit-testable (#1760 review). The traffic
+  // fallback to usefulness_score that used to be documented here was removed in
+  // #1927; see the note above trafficOf.
   function _toScatterPoints(nodes, favs) {
     return nodes
       .filter(n => n.role === 'repeater' || n.role === 'room')
@@ -2693,8 +2699,9 @@
         name: n.name || (n.public_key ? n.public_key.slice(0, 12) : '?'),
         role: n.role,
         fav: favs.has(n.public_key),
-        // #1456: prefer traffic_share_score, fall back to usefulness_score.
-        traffic: n.traffic_share_score != null ? n.traffic_share_score : (n.usefulness_score != null ? n.usefulness_score : null),
+        // #1927: see trafficOf above. A null drops the point from the scatter via
+        // the plottable filter, the same way a node with no bridge score is dropped.
+        traffic: n.traffic_share_score != null ? n.traffic_share_score : null,
         bridge: n.bridge_score != null ? n.bridge_score : null,
         relay1h: n.relay_count_1h != null ? n.relay_count_1h : null,
         relay24h: n.relay_count_24h != null ? n.relay_count_24h : null,

test-repeater-metric-scatter.js

@@ -81,8 +81,14 @@ assert(mapped[0].traffic === 0.5 && mapped[0].fav === true,
   'traffic_share_score is preferred and favorites are flagged');
 assert(mapped[0].bridge === 0.2 && mapped[0].relay1h === 1 && mapped[0].relay24h === 2 && mapped[0].adverts === 3,
   'bridge/relay/advert counts map onto their renamed point fields (advert_count → adverts)');
-assert(mapped[1].traffic === 0.07 && mapped[1].fav === false,
-  'missing traffic_share_score falls back to usefulness_score');
+// #1927: no fallback. usefulness_score is the #672 composite
+// (0.30*bridge + 0.25*coverage + ...), a different metric from the single
+// traffic axis, and substituting it put a composite under a column and an axis
+// that both promise share of non-advert traffic. A node carrying only the
+// composite now reads as unknown on that axis and is dropped from the plot by
+// the plottable filter, which is what #1927 asked for.
+assert(mapped[1].traffic === null && mapped[1].fav === false,
+  'a node with only usefulness_score has no traffic value; it must not be substituted');
 assert(mapped[2].traffic === null && mapped[2].bridge === null && mapped[2].relay1h === null,
   'rows without scores map to null (not 0/undefined) so plots can skip them');
 assert(mapped[3].name === 'NAMELESS0000',