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/srtt.rs

@@ -100,6 +100,14 @@ impl SrttCache {
         addrs.sort_by_key(|a| self.get(a.ip()));
     }
 
+    pub fn heap_bytes(&self) -> usize {
+        let per_slot = std::mem::size_of::<u64>()
+            + std::mem::size_of::<IpAddr>()
+            + std::mem::size_of::<SrttEntry>()
+            + 1;
+        self.entries.capacity() * per_slot
+    }
+
     pub fn len(&self) -> usize {
         self.entries.len()
     }
@@ -303,6 +311,16 @@ mod tests {
         assert_eq!(addrs, vec![sock(1), sock(2)]);
     }
 
+    #[test]
+    fn heap_bytes_grows_with_entries() {
+        let mut cache = SrttCache::new(true);
+        let empty = cache.heap_bytes();
+        for i in 1..=10u8 {
+            cache.record_rtt(ip(i), 100, false);
+        }
+        assert!(cache.heap_bytes() > empty);
+    }
+
     #[test]
     fn eviction_removes_oldest() {
         let mut cache = SrttCache::new(true);