feat(stats): per-container cpu/mem load (#1424 p3)

C0NT41N3R L04D on the SYST3M tab + GET /api/container-resources.

Backend (hive-c0re/src/container_stats.rs): reads cgroup v2 cpu.stat +
memory.{current,peak,max} for each running agent machine
(machine-h\x2d<name>.scope under machine.slice), read-only/world-
readable so no hive-priv. CPU is a two-sample (~200ms) host-normalised
percentage; one shared sleep covers all agents. Skips agents whose
scope dir is absent (= not running). Network omitted: agents share the
host netns, no per-container counter.

Frontend: a polled C0NT41N3R L04D table on SYST3M (agent / cpu / mem /
peak / limit with meter bars), reusing the ST4TS table style. Polls
/api/container-resources every 5s only while the tab is active.

Backend reviewed-in-principle by damocles (path escaping + cpu delta
math); ping for the on-host sign-off.

hive-c0re/src/lib.rs

@@ -23,6 +23,7 @@ pub mod broker;
 pub mod build_logs;
 pub mod capabilities;
 pub mod client;
+pub mod container_stats;
 pub mod container_view;
 pub mod coordinator;
 pub mod crash_watch;