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.

hive-log/README.md

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+# hive-log
+
+One shared log-sink chooser for the hyperhive daemons: `hive_log::init()`
+builds the `EnvFilter` every binary built before, then installs **exactly
+one** sink.
+
+## Why one sink and not both
+
+Under a systemd unit the process's stdout already **is** the journal.
+Adding a journald layer on top of the `fmt` layer therefore stores every
+record twice — once as the native journal entry, once as the text line
+`fmt` wrote to a descriptor that leads to the same journal.
+
+Writing to stdout also throws the severity away. A stdout line carries no
+priority field, so journald files the whole stream at one level and the
+swarm log store shows `info` whatever level `tracing` gave the record.
+Logging natively carries the level across. That is the reason to prefer
+journald where a journal exists, and the reason never to run both layers.
+
+## How the choice happens
+
+systemd sets `$JOURNAL_STREAM` to the device and inode numbers of the
+journal-connected descriptor, in decimal, separated by a colon.
+
+Checking that the variable exists is **not** enough. A child process
+inherits `$JOURNAL_STREAM` from its parent even when the parent redirected
+that child's stdout somewhere else, so presence alone reports a journal
+that the process does not actually write to. `hive-log` instead stats
+stdout — the descriptor the `fmt` layer writes to by default, and the one
+a journald layer would double up on — and compares both numbers against
+the parsed pair.
+
+A match installs the native journald layer alone. No match, an unset
+variable, or a value that does not parse installs the `fmt` layer alone.
+
+## When the journald layer fails to build
+
+`tracing_journald::layer()` validates the journal socket up front and
+returns an `io::Result`, so a missing journal surfaces as an error rather
+than a silent no-op. On that error `hive-log` installs the `fmt` layer and
+emits one warning through it. A process must never fail to start because
+of its logger, and a logger that drops every record in silence is worse
+than a degraded one that says so.
+
+## Scope
+
+The sink chooser and its tests. Nothing else belongs here: the crate
+exists so three binaries share one copy of this decision, not as a place
+for general utilities.