resource_limits: read the override map once per SSE scan

`container_view::build_all` renders every container on every scan, and
each agent's row resolved its limits through `effective()`, which reads
and parses `resource-limits.json` from disk. That is one file read per
agent per scan of a file that is identical for all of them.

Split the resolution in two: `effective_from` takes an already-loaded
map, and `effective` keeps the read-then-resolve shape for the
single-agent callers (`write_dropins`, which runs once per spawn and
rebuild and has no map to hand). `build_all` now loads the map once at
the top — the same treatment `topology::read()` already gets there —
and calls `effective_from` per agent.

No behaviour change: the fallback matrix lives in `resolve`, which both
paths still go through, and its tests are untouched.

The now-redundant `limits_for` is gone; `effective_from` covers its one
caller.

hive-c0re/src/agent_config/resource_limits.rs

@@ -70,20 +70,34 @@ pub fn read() -> BTreeMap<String, AgentLimits> {
     serde_json::from_str(&raw).unwrap_or_default()
 }
 
-/// Look up one agent's overrides. Returns the all-`None` default when
-/// the agent has no entry.
-#[must_use]
-pub fn limits_for(name: &str) -> AgentLimits {
-    read().get(name).cloned().unwrap_or_default()
-}
-
 /// Resolve the effective values for an agent, filling each unset field
 /// from the hive-wide default. This is the single place the fallback
 /// rule lives; `write_dropins` calls it and passes the result straight
 /// to systemd.
+///
+/// Reads the override file. Use [`effective_from`] when resolving more
+/// than one agent in a row.
 #[must_use]
 pub fn effective(name: &str, hive_cpu_quota: &str, hive_memory_max: &str) -> (String, String) {
-    resolve(&limits_for(name), hive_cpu_quota, hive_memory_max)
+    effective_from(&read(), name, hive_cpu_quota, hive_memory_max)
+}
+
+/// [`effective`] against an already-loaded map — the multi-agent form.
+///
+/// `container_view::build_all` renders every agent on each SSE scan, so
+/// it loads the map once and calls this per agent rather than re-reading
+/// the same small file N times per scan.
+#[must_use]
+pub fn effective_from(
+    limits: &BTreeMap<String, AgentLimits>,
+    name: &str,
+    hive_cpu_quota: &str,
+    hive_memory_max: &str,
+) -> (String, String) {
+    match limits.get(name) {
+        Some(l) => resolve(l, hive_cpu_quota, hive_memory_max),
+        None => (hive_cpu_quota.to_owned(), hive_memory_max.to_owned()),
+    }
 }
 
 /// Pure core of [`effective`], split out so the fallback matrix is