fix(ingestor): warn when TRACE payload decode fails but observation stored (closes #889) (#992)

## Summary

Closes #889.

When a TRACE packet's payload is too short to decode (< 9 bytes),
`decodeTrace` returns an error in `Payload.Error` but the observation is
still stored with empty `Path.Hops`. Previously this was completely
silent — no log, no anomaly flag, no indication the row is degraded.

This fix populates `DecodedPacket.Anomaly` with the decode error message
(e.g., `"TRACE payload decode failed: too short"`) so operators and
downstream consumers can identify degraded observations.

## TDD Commit History

1. **Red commit** `04e0165` — failing test asserting `Anomaly` is set
when TRACE payload decode fails
2. **Green commit** `d3e72d1` — 3-line fix in `decoder.go` line 601-603:
check `payload.Error != ""` for TRACE packets and set anomaly

## What Changed

`cmd/ingestor/decoder.go` (lines 601-603): Added a check before the
existing TRACE path-parsing block. If `payload.Error` is non-empty for a
TRACE packet, `anomaly` is set to `"TRACE payload decode failed:
<error>"`.

`cmd/ingestor/decoder_test.go`: Added
`TestDecodeTracePayloadFailSetsAnomaly` — constructs a TRACE packet with
a 4-byte payload (too short), asserts the packet is still returned
(observation stored) and `Anomaly` is populated.

## Verification

- `go build ./...` ✓
- `go test ./...` ✓ (all pass including new test)
- Anti-tautology: reverting the fix causes the new test to fail (asserts
`pkt.Anomaly == ""` → error)

---------

Co-authored-by: you <you@example.com>

cmd/ingestor/decoder.go

@@ -599,6 +599,9 @@ func DecodePacket(hexString string, channelKeys map[string]string, validateSigna
 	// We expose hopsCompleted (count of SNR bytes) so consumers can distinguish
 	// how far the trace got vs the full intended route.
 	var anomaly string
+	if header.PayloadType == PayloadTRACE && payload.Error != "" {
+		anomaly = fmt.Sprintf("TRACE payload decode failed: %s", payload.Error)
+	}
 	if header.PayloadType == PayloadTRACE && payload.PathData != "" {
 		// Flag anomalous routing — firmware only sends TRACE as DIRECT
 		if header.RouteType != RouteDirect && header.RouteType != RouteTransportDirect {

cmd/ingestor/decoder_test.go

@@ -1926,3 +1926,24 @@ func TestDecodePathFromRawHex_Transport(t *testing.T) {
 		}
 	}
 }
+
+func TestDecodeTracePayloadFailSetsAnomaly(t *testing.T) {
+	// Issue #889: TRACE packet with payload too short to decode (< 9 bytes)
+	// should still return a DecodedPacket (observation stored) but with Anomaly
+	// set to warn operators that the decode was degraded.
+	// Packet: header 0x26 (TRACE+DIRECT), pathByte 0x00, payload 4 bytes (too short).
+	pkt, err := DecodePacket("2600aabbccdd", nil, false)
+	if err != nil {
+		t.Fatalf("DecodePacket error: %v", err)
+	}
+	if pkt.Payload.Type != "TRACE" {
+		t.Fatalf("payload type=%s, want TRACE", pkt.Payload.Type)
+	}
+	if pkt.Payload.Error == "" {
+		t.Fatal("expected payload.Error to indicate decode failure")
+	}
+	// The key assertion: Anomaly must be set when TRACE decode fails
+	if pkt.Anomaly == "" {
+		t.Error("expected Anomaly to be set when TRACE payload decode fails but observation is stored")
+	}
+}