hive-sh4re: split inbox, container, journal, and schedule wire shapes into their own modules

Closes the #3110 split — lib.rs is now just the crate doc comment and
the pub mod list.

journal.rs's new doc comment fixes a pre-existing bug: the old
JournalPriority doc text in lib.rs was actually half Capability's doc
(a leftover from an earlier reorder that moved the code but not the
comment above it).

hive-sh4re/src/journal.rs

@@ -0,0 +1,41 @@
+//! Journal-priority encoding for `GetHostJournal` (`read_host_journal`
+//! capability). One small enum, own topic module rather than a
+//! catch-all — see `hive-sh4re/README.md`'s module list.
+
+use serde::{Deserialize, Serialize};
+
+/// Syslog priority levels for `GetHostJournal`. Serialised as lowercase
+/// strings matching journalctl `-p` accepted values.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize, schemars::JsonSchema)]
+#[serde(rename_all = "lowercase")]
+pub enum JournalPriority {
+    Emerg,
+    Alert,
+    Crit,
+    Err,
+    Warning,
+    Notice,
+    Info,
+    Debug,
+}
+
+impl JournalPriority {
+    /// Returns the lowercase string journalctl expects for `-p`. Named
+    /// `as_journald_str` (not `as_str`) because this is a specific
+    /// external-tool encoding, not the enum's general wire/db string —
+    /// distinct call sites shouldn't reach for this by accident when they
+    /// actually want the serde wire representation.
+    #[must_use]
+    pub fn as_journald_str(self) -> &'static str {
+        match self {
+            Self::Emerg => "emerg",
+            Self::Alert => "alert",
+            Self::Crit => "crit",
+            Self::Err => "err",
+            Self::Warning => "warning",
+            Self::Notice => "notice",
+            Self::Info => "info",
+            Self::Debug => "debug",
+        }
+    }
+}