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-priv-sock/src/lib.rs

@@ -343,15 +343,17 @@ pub enum PrivRequest {
         container: String,
         memory_max: String,
         cpu_quota: String,
-        /// cgroup v2 `cpu.weight`, 1..=10000. `#[serde(default)]` so a
-        /// hive-priv built before this field still deserialises new
-        /// requests; 0 is treated as "omit the line" by the writer.
+        /// cgroup v2 `cpu.weight`, 1..=10000. `None` means "not
+        /// configured" — the writer omits the line entirely, leaving the
+        /// kernel default. `#[serde(default)]` so a request from a
+        /// hive-c0re built before this field existed deserialises to
+        /// `None` and reproduces the pre-weights drop-in.
         #[serde(default)]
-        cpu_weight: u32,
-        /// cgroup v2 `io.weight`, 1..=10000. Same default/omit rule as
+        cpu_weight: Option<u32>,
+        /// cgroup v2 `io.weight`, 1..=10000. Same `None` = omit rule as
         /// `cpu_weight`.
         #[serde(default)]
-        io_weight: u32,
+        io_weight: Option<u32>,
     },
 
     /// Remove `/run/systemd/system/container@<container>.service.d/` if present.