log: send records natively to journald, keep stdout off-unit

A record written to stdout carries no priority, so journald files the
whole stream at one level and the swarm log store shows `info` whatever
level `tracing` gave it. Under a systemd unit the process's stdout
already *is* the journal, so the fix is to speak the journal protocol
directly and let each record carry its own severity.

New `hive-log` crate holds the one sink chooser, called by `hive-c0re`,
`hive-agent` and `swarm-controller`. It builds the same `EnvFilter`
those binaries always built, then installs exactly one layer — never
both, since a journald layer stacked on the `fmt` layer under a unit
stores every record twice.

The choice is an fstat compare, not a presence test: a child inherits
`$JOURNAL_STREAM` even when its own stdout was redirected elsewhere, so
the variable existing proves nothing. The crate parses `dev:inode` out
of it and compares both numbers against an fstat of stdout, the
descriptor the `fmt` layer writes to by default. No match, unset, or
unparseable takes the `fmt` branch. A journald layer that fails to
construct despite a match falls back to `fmt` and warns through it —
a process must never fail to start because of its logger.

CLAUDE.md

@@ -72,6 +72,14 @@ hand-maintained per-file tree drifts out of sync with the code.
   remix. `GraphWire::wire_snapshot` is blanket-implemented for any
   `Graph<N, R>` whose parameters implement both — so a payload that has
   never said how it displays cannot reach a viewer at all.
+- **`hive-log/`** — the one log-sink chooser, called by `hive-c0re`,
+  `hive-agent` and `swarm-controller` and by nothing else. `init()` builds
+  the `EnvFilter` those binaries always built, then installs **exactly
+  one** sink: the native journald layer when an `fstat` of stdout matches
+  the device and inode numbers systemd exports in `$JOURNAL_STREAM`, the
+  `fmt` stdout layer otherwise. Under a unit stdout already **is** the
+  journal, so both layers together store every record twice. Deliberately
+  single-purpose — do not grow it into a utility crate.
 - **`hive-screen-mcp/`** — stdio MCP bridge for GUI agents
   (`hyperhive.gui.enable`): `screenshot` via `grim`, `type_text` /
   `key_press` via `wtype` (Wayland virtual-keyboard protocol), and