feat: add memory footprint to /stats and dashboard (#26)

* feat: add memory footprint to /stats and dashboard

Per-structure heap estimation (cache, blocklist, query log, SRTT,
overrides) with process RSS via mach_task_basic_info / sysconf.
Dashboard gets a 6th stat card and a sidebar breakdown panel with
stacked bar visualization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: use phys_footprint on macOS to match Activity Monitor

Switch from MACH_TASK_BASIC_INFO (resident_size) to TASK_VM_INFO
(phys_footprint) which matches Activity Monitor's Memory column.
Also: capacity-aware heap estimation, entry counts in memory payload,
heap_bytes tests for all stores.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: remove redundant fields and fix naming in memory stats

Remove duplicate entry counts from MemoryStats (already in parent
StatsResponse), rename process_rss_bytes to process_memory_bytes
to match macOS phys_footprint semantics, drop restating comments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/packet.rs

@@ -66,6 +66,25 @@ impl DnsPacket {
         pkt
     }
 
+    pub fn heap_bytes(&self) -> usize {
+        fn records_heap(records: &[DnsRecord]) -> usize {
+            records
+                .iter()
+                .map(|r| std::mem::size_of::<DnsRecord>() + r.heap_bytes())
+                .sum::<usize>()
+        }
+        let questions: usize = self
+            .questions
+            .iter()
+            .map(|q| std::mem::size_of::<DnsQuestion>() + q.name.capacity())
+            .sum();
+        questions
+            + records_heap(&self.answers)
+            + records_heap(&self.authorities)
+            + records_heap(&self.resources)
+            + self.edns.as_ref().map_or(0, |e| e.options.capacity())
+    }
+
     pub fn response_from(query: &DnsPacket, rescode: crate::header::ResultCode) -> DnsPacket {
         let mut resp = DnsPacket::new();
         resp.header.id = query.header.id;
@@ -591,4 +610,16 @@ mod tests {
             panic!("expected DNSKEY");
         }
     }
+
+    #[test]
+    fn heap_bytes_accounts_for_records() {
+        let mut pkt = DnsPacket::new();
+        let empty = pkt.heap_bytes();
+        pkt.answers.push(DnsRecord::A {
+            domain: "example.com".into(),
+            addr: "1.2.3.4".parse().unwrap(),
+            ttl: 300,
+        });
+        assert!(pkt.heap_bytes() > empty);
+    }
 }