fix: hide hash size for zero-hop direct adverts (#649) (#653)

## Fix: Zero-hop DIRECT packets report bogus hash_size

Closes #649

### Problem
When a DIRECT packet has zero hops (pathByte lower 6 bits = 0), the
generic `hash_size = (pathByte >> 6) + 1` formula produces a bogus value
(1-4) instead of 0/unknown. This causes incorrect hash size displays and
analytics for zero-hop direct adverts.

### Solution

**Frontend (JS):**
- `packets.js` and `nodes.js` now check `(pathByte & 0x3F) === 0` to
detect zero-hop packets and suppress bogus hash_size display.

**Backend (Go):**
- Both `cmd/server/decoder.go` and `cmd/ingestor/decoder.go` reset
`HashSize=0` for DIRECT packets where `pathByte & 0x3F == 0` (hash_count
is zero).
- TRACE packets are excluded since they use hashSize to parse hop data
from the payload.
- The condition uses `pathByte & 0x3F == 0` (not `pathByte == 0x00`) to
correctly handle the case where hash_size bits are non-zero but
hash_count is zero — matching the JS frontend approach.

### Testing

**Backend:**
- Added 4 tests each in `cmd/server/decoder_test.go` and
`cmd/ingestor/decoder_test.go`:
  - DIRECT + pathByte 0x00 → HashSize=0 ✅
- DIRECT + pathByte 0x40 (hash_size bits set, hash_count=0) → HashSize=0
✅
  - Non-DIRECT + pathByte 0x00 → HashSize=1 (unchanged) ✅
  - DIRECT + pathByte 0x01 (1 hop) → HashSize=1 (unchanged) ✅
- All existing tests pass (`go test ./...` in both cmd/server and
cmd/ingestor)

**Frontend:**
- Verified hash size display is suppressed for zero-hop direct adverts

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: you <you@example.com>

cmd/server/decoder.go

@@ -388,6 +388,16 @@ func DecodePacket(hexString string) (*DecodedPacket, error) {
 		}
 	}
 
+	// Zero-hop direct packets have hash_count=0 (lower 6 bits of pathByte),
+	// which makes the generic formula yield a bogus hashSize. Reset to 0
+	// (unknown) so API consumers get correct data. We mask with 0x3F to check
+	// only hash_count, matching the JS frontend approach — the upper hash_size
+	// bits are meaningless when there are no hops. Skip TRACE packets — they
+	// use hashSize to parse hops from the payload above.
+	if (header.RouteType == RouteDirect || header.RouteType == RouteTransportDirect) && pathByte&0x3F == 0 && header.PayloadType != PayloadTRACE {
+		path.HashSize = 0
+	}
+
 	return &DecodedPacket{
 		Header:         header,
 		TransportCodes: tc,