Implements the locked spec from #1359.
Red commit: 68a140a8 — `distinctRelayCount` stub returns 0; test fails
on assertion (compiles + runs to assertion, not a build error).
Green commit: 48c2ddad — real implementation.
## Backend (in-memory, no SQL, no schema change)
- `cmd/server/relay_airtime_share.go`
- `distinctRelayCount(tx)` — unions the resolved-pubkey reverse index
for `tx.ID`. That index already dedups `(pubkey-hash, txID)` pairs
across every observation's `resolved_path`, so its length IS the count
of distinct repeaters that forwarded the packet. NOT length of any
single observation's resolved_path (the bug-trap from #1358).
- `computeRelayAirtimeShare(window)` — per-tx `score = payload_bytes ×
distinctRelays`, bucketed by `payload_type`, sorted desc by airtime_pct.
- `GetRelayAirtimeShareWithWindow` — cached behind existing `rfCache` +
`rfCacheTTL` pool. Shallow-copies the cached payload with `cached=true`
for the client.
- `cmd/server/routes.go` — `GET
/api/analytics/relay-airtime-share?window=…` returning
`{rows:[{payload_type,type,count,count_pct,score,airtime_pct}],
total_count, total_score, window, cached}`.
## Frontend
- `public/analytics.js`
- `renderRelayAirtimeDumbbell(data)` — horizontal dumbbell chart per
payload_type. Gray dot = count %, colored dot = airtime %, connector
line between them = the divergence, shared 0-100% axis, sorted desc by
airtime.
- Tooltip: payload_type, count %, count N, airtime %, raw score,
within-mesh caveat.
- Title: **Relay Airtime Share**.
- Subtitle (exact): `Score = payload bytes × distinct repeaters that
forwarded the packet. Counts relay re-transmissions; originator TX
excluded. Not comparable across meshes.`
- Mounted on the Overview tab immediately beneath Payload Type Mix.
## Tests
`TestRelayAirtimeShare_ADVERTvsACKDivergence` — the locked acceptance
scenario:
- 1 ADVERT (200 B, 8 distinct relays) → score 1600, airtime 100%
- 1000 ACKs (10 B, 0 relays each) → score 0, airtime 0%
- Count distribution is the inverse (ACK 99.9%, ADVERT 0.1%).
- Sort assertion: ADVERT is rows[0] by airtime_pct desc.
Full suite: `go test -short ./cmd/server/...` → PASS (25.9s).
## Acceptance criteria
- [x] In-memory `airtime_usage_score` accumulator in analytics path
- [x] `distinctRelayCount(tx)` helper unioning resolved-pubkey reverse
index across all observations of `transmission_id`
- [x] `/api/analytics/relay-airtime-share?window=…` endpoint
- [x] Cached via existing `rfCache` + `rfCacheTTL`; no new cache layer
- [x] Dumbbell chart on `/analytics` beneath Payload Type Mix;
gray=count, colored=airtime, shared axis, sorted desc by airtime
- [x] Title + subtitle exactly as specified
- [x] Tooltip with payload_type, count %, count N, airtime %, raw score,
caveat
- [x] Unit test demonstrates the ADVERT-vs-ACK divergence
- [x] No new SQL, no new index, no schema migration (verified via diff)
- [ ] Live staging bench (<5ms p99 uncached / <1ms cached) — deferred to
follow-up; cached behind 60s `rfCacheTTL` so steady-state cost is a map
lookup
## Preflight overrides
- Branch scope cross-stack: justified — backend endpoint and frontend
chart are a single deliverable per #1359 spec (one chart bound to one
endpoint, no incremental staging).
Fixes#1359
---------
Co-authored-by: bot <bot@local>
