refactor(#2754): make the container weights Option, not a 0 sentinel

Encoding "not configured" as weight 0 worked (the writer omitted the
line) but the type lied: 0 is not a legal cgroup v2 weight, and every
reader had to know the sentinel. Use Option<u32> end to end instead —
wire type, priv_client, HiveEnv, drop-in writer — so "unset" is a state
of the type rather than a magic value.

The nix options become nullOr, keeping their default of 80; null now
expresses "leave the setting out of the drop-in entirely" declaratively,
which is the useful shape on a host whose IO scheduler ignores
io.weight anyway.

Backward compat is unchanged: the fields stay #[serde(default)], so a
request from an older hive-c0re deserialises to None and reproduces the
pre-weights drop-in byte for byte. The test that pins that now passes
None instead of 0.

hive-c0re/src/coordinator.rs

@@ -87,12 +87,14 @@ pub struct Coordinator {
     pub agent_memory_max: String,
     /// Per-agent systemd `CPUWeight=` (cgroup v2 `cpu.weight`, 1..=10000).
     /// Same drop-in. A *relative share* under contention, not a cap — see
-    /// the `agentCpuWeight` NixOS option for the full semantics.
-    pub agent_cpu_weight: u32,
+    /// the `agentCpuWeight` NixOS option for the full semantics. `None`
+    /// (option set to `null`) omits the setting: kernel default, and the
+    /// drop-in is byte-identical to the one written before weights existed.
+    pub agent_cpu_weight: Option<u32>,
     /// Per-agent systemd `IOWeight=` (cgroup v2 `io.weight`, 1..=10000).
-    /// Same drop-in, same relative-share semantics as
+    /// Same drop-in, same relative-share and `None` semantics as
     /// [`Self::agent_cpu_weight`].
-    pub agent_io_weight: u32,
+    pub agent_io_weight: Option<u32>,
     /// Operator-tunable model→price table backing the hive-wide cost
     /// estimate on the ST4TS tab. Set via `services.hyperhive.modelPrices`
     /// and passed to `hive-c0re serve --model-prices <json>`. Models not
@@ -220,10 +222,11 @@ pub struct HiveEnv {
     pub agent_memory_max: String,
     /// Per-agent systemd `CPUWeight=` — cgroup v2 `cpu.weight`, 1..=10000.
     /// Relative share under contention, not a cap; see `agentCpuWeight`.
-    pub agent_cpu_weight: u32,
+    /// `None` = leave the setting out of the drop-in (kernel default).
+    pub agent_cpu_weight: Option<u32>,
     /// Per-agent systemd `IOWeight=` — cgroup v2 `io.weight`, 1..=10000.
     /// Same semantics as [`Self::agent_cpu_weight`]; see `agentIoWeight`.
-    pub agent_io_weight: u32,
+    pub agent_io_weight: Option<u32>,
 }
 
 impl Default for HiveEnv {
@@ -244,8 +247,8 @@ impl Default for HiveEnv {
             // Slightly below the kernel default of 100, so agent
             // containers yield to host services + the infra containers
             // (which carry no drop-in and stay at 100) under contention.
-            agent_cpu_weight: 80,
-            agent_io_weight: 80,
+            agent_cpu_weight: Some(80),
+            agent_io_weight: Some(80),
         }
     }
 }

hive-c0re/src/lifecycle/host_config.rs

@@ -45,12 +45,14 @@ pub async fn write_dropins(name: &str, hive: &HiveEnv, paths: &AgentPaths) -> Re
 /// The weights are hive-wide (`services.hyperhive.agentCpuWeight` /
 /// `agentIoWeight`) — unlike the caps they have no per-agent override in
 /// `meta/resource-limits.json`, so they come straight off [`HiveEnv`].
+/// `None` (the nix option set to `null`) means the weight line is left out
+/// and the container keeps the kernel default.
 async fn set_resource_limits(
     container: &str,
     cpu_quota: &str,
     memory_max: &str,
-    cpu_weight: u32,
-    io_weight: u32,
+    cpu_weight: Option<u32>,
+    io_weight: Option<u32>,
 ) -> Result<()> {
     crate::priv_client::write_resource_limits(
         container, memory_max, cpu_quota, cpu_weight, io_weight,

hive-c0re/src/main.rs

@@ -197,11 +197,15 @@ async fn main() -> Result<()> {
             if let Some(v) = agent_memory_max {
                 sc.env.agent_memory_max = v;
             }
+            // Passing the flag sets a weight; omitting it keeps whatever the
+            // config file says (including `null` = don't emit the setting).
+            // There's deliberately no flag spelling for "clear it" — that's
+            // what the nix option's `null` is for.
             if let Some(v) = agent_cpu_weight {
-                sc.env.agent_cpu_weight = v;
+                sc.env.agent_cpu_weight = Some(v);
             }
             if let Some(v) = agent_io_weight {
-                sc.env.agent_io_weight = v;
+                sc.env.agent_io_weight = Some(v);
             }
             if let Some(v) = model_prices {
                 sc.model_prices =

hive-c0re/src/priv_client.rs

@@ -174,8 +174,8 @@ pub async fn write_resource_limits(
     container: &str,
     memory_max: &str,
     cpu_quota: &str,
-    cpu_weight: u32,
-    io_weight: u32,
+    cpu_weight: Option<u32>,
+    io_weight: Option<u32>,
 ) -> Result<()> {
     ok(call(&PrivRequest::WriteResourceLimits {
         container: container.to_owned(),