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/cache.rs

@@ -142,6 +142,26 @@ impl DnsCache {
         self.entry_count = 0;
     }
 
+    pub fn heap_bytes(&self) -> usize {
+        let outer_slot = std::mem::size_of::<u64>()
+            + std::mem::size_of::<String>()
+            + std::mem::size_of::<HashMap<QueryType, CacheEntry>>()
+            + 1;
+        let mut total = self.entries.capacity() * outer_slot;
+        for (domain, type_map) in &self.entries {
+            total += domain.capacity();
+            let inner_slot = std::mem::size_of::<u64>()
+                + std::mem::size_of::<QueryType>()
+                + std::mem::size_of::<CacheEntry>()
+                + 1;
+            total += type_map.capacity() * inner_slot;
+            for entry in type_map.values() {
+                total += entry.packet.heap_bytes();
+            }
+        }
+        total
+    }
+
     pub fn remove(&mut self, domain: &str) {
         let domain_lower = domain.to_lowercase();
         if let Some(type_map) = self.entries.remove(&domain_lower) {
@@ -194,3 +214,23 @@ fn adjust_ttls(records: &mut [DnsRecord], new_ttl: u32) {
         record.set_ttl(new_ttl);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::packet::DnsPacket;
+
+    #[test]
+    fn heap_bytes_grows_with_entries() {
+        let mut cache = DnsCache::new(100, 1, 3600);
+        let empty = cache.heap_bytes();
+        let mut pkt = DnsPacket::new();
+        pkt.answers.push(DnsRecord::A {
+            domain: "example.com".into(),
+            addr: "1.2.3.4".parse().unwrap(),
+            ttl: 300,
+        });
+        cache.insert("example.com", QueryType::A, &pkt);
+        assert!(cache.heap_bytes() > empty);
+    }
+}