feat: fill in the RFC 5424 header fields

[DavidCozens]

The record so far carries no timestamp and no device name. Fill the RFC 5424 header fields from
what the device already has: the clock it acquired at boot, the address on its interface, and its
own name.

struct SolidSyslogConfig config = {
    /* ... */
    .Clock       = SyslogFields_Clock,
    .GetHostname = SyslogFields_Hostname,
    .GetAppName  = SyslogFields_AppName,
};

<134>1 2026-08-16T19:17:54.430000Z 10.0.2.15 solid-syslog-example - BOOT - device started

PROCID stays nil, because a bare-metal image has no process to identify, and so does
STRUCTURED-DATA until the next stage.

Two things the adapters have to get right. The timestamp struct is zeroed before it is filled, so a
clock that cannot answer leaves Month == 0, fails the library's validation, and is emitted as the
nil value rather than as a wrong time. And the hostname is read under the lwIP core lock, with
ip4addr_ntoa_r rather than ip4addr_ntoa — the latter shares one static buffer across callers.

Where a device has no resolvable name, RFC 5424 section 6.2.4 allows its address in the HOSTNAME
field instead, which is this device exactly.

When you need it. As soon as more than one device reports to the collector, or a record's time
will be relied on. Everything the later stages add — a sequence number, the clock's quality, the
device's own identity — builds on these fields rather than replacing them.

[DavidCozens]

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