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/record.rs

@@ -136,6 +136,46 @@ impl DnsRecord {
         }
     }
 
+    pub fn heap_bytes(&self) -> usize {
+        match self {
+            DnsRecord::A { domain, .. } => domain.capacity(),
+            DnsRecord::NS { domain, host, .. } | DnsRecord::CNAME { domain, host, .. } => {
+                domain.capacity() + host.capacity()
+            }
+            DnsRecord::MX { domain, host, .. } => domain.capacity() + host.capacity(),
+            DnsRecord::AAAA { domain, .. } => domain.capacity(),
+            DnsRecord::DNSKEY {
+                domain, public_key, ..
+            } => domain.capacity() + public_key.capacity(),
+            DnsRecord::DS { domain, digest, .. } => domain.capacity() + digest.capacity(),
+            DnsRecord::RRSIG {
+                domain,
+                signer_name,
+                signature,
+                ..
+            } => domain.capacity() + signer_name.capacity() + signature.capacity(),
+            DnsRecord::NSEC {
+                domain,
+                next_domain,
+                type_bitmap,
+                ..
+            } => domain.capacity() + next_domain.capacity() + type_bitmap.capacity(),
+            DnsRecord::NSEC3 {
+                domain,
+                salt,
+                next_hashed_owner,
+                type_bitmap,
+                ..
+            } => {
+                domain.capacity()
+                    + salt.capacity()
+                    + next_hashed_owner.capacity()
+                    + type_bitmap.capacity()
+            }
+            DnsRecord::UNKNOWN { domain, data, .. } => domain.capacity() + data.capacity(),
+        }
+    }
+
     pub fn set_ttl(&mut self, new_ttl: u32) {
         match self {
             DnsRecord::A { ttl, .. }
@@ -650,4 +690,14 @@ mod tests {
         let parsed = round_trip(&rec);
         assert_eq!(rec, parsed);
     }
+
+    #[test]
+    fn heap_bytes_reflects_string_capacity() {
+        let rec = DnsRecord::CNAME {
+            domain: "a]".repeat(100),
+            host: "b".repeat(200),
+            ttl: 60,
+        };
+        assert!(rec.heap_bytes() >= 300);
+    }
 }