hive-c0re: address review on btrfs qgroup usage parsing

Select the level-0 (`0/<subvolid>`) leaf qgroup row explicitly instead
of taking the last data line, so usage parsing is unambiguous even if an
operator has assigned the subvolume to a higher-level aggregate qgroup.
`btrfs qgroup show -f <path>` already scopes the listing to qgroups
impacting the given path (excluding ancestral qgroups, per
btrfs-qgroup-show(8)); selecting the `0/` leaf among them pins it to the
subvolume's own automatic usage qgroup.

Also: case-insensitive match on the stable "quota not enabled" error
fragment (wording varies across btrfs-progs versions), `# Errors` doc
sections on the three public priv_client quota functions, and precise
doc comments on the `-f` flag semantics.

hive-priv/src/main.rs

@@ -671,10 +671,12 @@ async fn ensure_btrfs_quota() -> Result<(String, String)> {
     Ok((String::new(), String::new()))
 }
 
-/// `ReadSubvolumeUsage` — return an agent subvolume's qgroup row
+/// `ReadSubvolumeUsage` — return an agent subvolume's qgroup rows
 /// (`btrfs qgroup show -f --raw <…/agent_name>`) verbatim in stdout for
-/// hive-c0re to parse. `-f` filters to the subvolume the path lives in, so
-/// the output is the single relevant row. See the wire doc.
+/// hive-c0re to parse. `-f` lists the qgroups impacting the given path,
+/// excluding ancestral qgroups (per btrfs-qgroup-show(8)) — so it scopes
+/// to this subvolume and never mixes in other agents'. hive-c0re then
+/// selects the level-0 (`0/<subvolid>`) leaf row. See the wire doc.
 async fn read_subvolume_usage(agent_name: &str) -> Result<(String, String)> {
     let agent_root = PathBuf::from(AGENT_STATE_ROOT).join(agent_name);
     let out = Command::new("btrfs")