mimalloc for ipx and the directory, and metrics and traces for memory (#224)

ipx's resident memory climbed from about 40 MB to 500-850 MB each night while scans ran five
times as often, and stayed there until a restart; the directory sat at 1.25 GiB idle. glibc's
allocator keeps what each thread's arena freed. mimalloc gives freed pages back, so resident
memory now follows what is in use. jemalloc was the first choice for its in-use figures, but its
upstream was archived in 2025.

Declared once in the directory crate, which ipx builds in, so both binaries get it.

What was missing to see it: the directory served no /metrics (now on metrics_bind, a port apart
from the tunnel's), nothing counted the feed bytes fetched, and the fetch span had no body size.
The feed crate's spans never reached Tempo either: both trace exporters filtered to their own
crate, so ipx_feed's fetch and parse spans were dropped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/CHANGELOG.md

@@ -10,8 +10,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- With `metrics_bind` set, the directory serves Prometheus metrics on a port of its own: memory, open files, and the feeds it has fetched and their bytes. A feed check's trace says how large the feed was (#224).
+
 ### Changed
 
+- The directory uses mimalloc instead of glibc's allocator, so it gives back memory it has freed instead of holding over a gigabyte while idle (#224).
 - The example in docs/directory.md has no healthcheck; `ipx-directory health` is still there to run by hand (#218).
 
 ## [1.0.0] - 2026-10-12