fix: perf page shows tracked memory instead of heap allocation (#718)

## Summary

The perf page "Memory Used" tile displayed `estimatedMB` (Go
`runtime.HeapAlloc`), which includes all Go runtime allocations — not
just packet store data. This made the displayed value misleading: it
showed ~2.4GB heap when only ~833MB was actual tracked packet data.

## Changes

### Frontend (`public/perf.js`)
- Primary tile now shows `trackedMB` as **"Tracked Memory"** — the
self-accounted packet store memory
- Added separate **"Heap (debug)"** tile showing `estimatedMB` for
runtime visibility

### Backend
- **`types.go`**: Added `TrackedMB` field to `HealthPacketStoreStats`
struct
- **`routes.go`**: Populate `TrackedMB` in `/health` endpoint response
from `GetPerfStoreStatsTyped()`
- **`routes_test.go`**: Assert `trackedMB` exists in health endpoint's
`packetStore`
- **`testdata/golden/shapes.json`**: Updated shape fixture with new
field

### What was already correct
- `/api/perf/stats` already exposed both `estimatedMB` and `trackedMB`
- `trackedMemoryMB()` method already existed in store.go
- Eviction logic already used `trackedBytes` (not HeapAlloc)

## Testing
- All Go tests pass (`go test ./... -count=1`)
- No frontend logic changes beyond template string field swap

Fixes #717

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

cmd/server/routes.go

@@ -446,10 +446,12 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	// Real packet store stats
 	pktCount := 0
 	var pktEstMB float64
+	var pktTrackedMB float64
 	if s.store != nil {
 		ps := s.store.GetPerfStoreStatsTyped()
 		pktCount = ps.TotalLoaded
 		pktEstMB = ps.EstimatedMB
+		pktTrackedMB = ps.TrackedMB
 	}
 
 	// Real cache stats
@@ -515,6 +517,7 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 		PacketStore: HealthPacketStoreStats{
 			Packets:     pktCount,
 			EstimatedMB: pktEstMB,
+			TrackedMB:   pktTrackedMB,
 		},
 		Perf: HealthPerfStats{
 			TotalRequests: int(perfRequests),

cmd/server/routes_test.go

@@ -170,6 +170,9 @@ func TestHealthEndpoint(t *testing.T) {
 	if _, ok := pktStore["estimatedMB"]; !ok {
 		t.Error("expected estimatedMB in packetStore")
 	}
+	if _, ok := pktStore["trackedMB"]; !ok {
+		t.Error("expected trackedMB in packetStore")
+	}
 
 	// Verify eventLoop (GC pause metrics matching Node.js shape)
 	el, ok := body["eventLoop"].(map[string]interface{})

cmd/server/testdata/golden/shapes.json

@@ -916,6 +916,9 @@
           },
           "estimatedMB": {
             "type": "number"
+          },
+          "trackedMB": {
+            "type": "number"
           }
         }
       },

cmd/server/types.go

@@ -115,6 +115,7 @@ type WebSocketStatsResp struct {
 type HealthPacketStoreStats struct {
 	Packets     int     `json:"packets"`
 	EstimatedMB float64 `json:"estimatedMB"`
+	TrackedMB   float64 `json:"trackedMB"`
 }
 
 type SlowQuery struct {

public/perf.js

@@ -91,8 +91,9 @@
         const ps = server.packetStore;
         html += `<h3>In-Memory Packet Store</h3><div style="display:flex;gap:16px;flex-wrap:wrap;margin:8px 0;">
           <div class="perf-card"><div class="perf-num">${ps.inMemory.toLocaleString()}</div><div class="perf-label">Packets in RAM</div></div>
-          <div class="perf-card"><div class="perf-num">${ps.estimatedMB}MB</div><div class="perf-label">Memory Used</div></div>
+          <div class="perf-card"><div class="perf-num">${ps.trackedMB}MB</div><div class="perf-label">Tracked Memory</div></div>
           <div class="perf-card"><div class="perf-num">${ps.maxMB}MB</div><div class="perf-label">Memory Limit</div></div>
+          <div class="perf-card"><div class="perf-num">${ps.estimatedMB}MB</div><div class="perf-label">Heap (debug)</div></div>
           <div class="perf-card"><div class="perf-num">${ps.queries.toLocaleString()}</div><div class="perf-label">Queries Served</div></div>
           <div class="perf-card"><div class="perf-num">${ps.inserts.toLocaleString()}</div><div class="perf-label">Live Inserts</div></div>
           <div class="perf-card"><div class="perf-num">${ps.evicted.toLocaleString()}</div><div class="perf-label">Evicted</div></div>