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/override_store.rs

@@ -117,6 +117,22 @@ impl OverrideStore {
         self.entries.clear();
     }
 
+    pub fn heap_bytes(&self) -> usize {
+        let per_slot = std::mem::size_of::<u64>()
+            + std::mem::size_of::<String>()
+            + std::mem::size_of::<OverrideEntry>()
+            + 1;
+        let table = self.entries.capacity() * per_slot;
+        let heap: usize = self
+            .entries
+            .iter()
+            .map(|(k, v)| {
+                k.capacity() + v.domain.capacity() + v.target.capacity() + v.record.heap_bytes()
+            })
+            .sum();
+        table + heap
+    }
+
     pub fn active_count(&self) -> usize {
         self.entries.values().filter(|e| !e.is_expired()).count()
     }
@@ -154,3 +170,16 @@ fn parse_target(domain: &str, target: &str, ttl: u32) -> Result<(QueryType, DnsR
         },
     ))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn heap_bytes_grows_with_entries() {
+        let mut store = OverrideStore::new();
+        let empty = store.heap_bytes();
+        store.insert("example.com", "1.2.3.4", 300, None).unwrap();
+        assert!(store.heap_bytes() > empty);
+    }
+}