feat: buffer records and drain them on the service task

[DavidCozens]

A SolidSyslogCircularBuffer between SolidSyslog_Log and the sender, drained by a service task
calling SolidSyslog_Service. SolidSyslog_Log formats, enqueues and returns; the service task
does the I/O.

static uint8_t s_ring[SOLIDSYSLOG_CIRCULAR_BUFFER_RING_BYTES(SYSLOG_BUFFER_RECORDS)];

.Buffer = SolidSyslogCircularBuffer_Create(SolidSyslogFreeRtosMutex_Create(), s_ring, sizeof(s_ring)),

This separates logging an event from sending it. SolidSyslog_Log becomes safe to call from any
number of tasks, and cheap enough to call from the place the event actually happens rather than
from somewhere convenient later. Nothing that logs waits on the network.

A whole record now passes through both seams, and the depth follows it. The log task formats
one; the service task drains one and sends it. Sized at the RTOS floor, the service task did not
merely trip its overflow hook — it locked the CPU up, because a frame that large clears the guard
band entirely rather than growing into it. Both seams now hold a record and their onward call, and
both are tightened against measured high-water marks at the end.

The mutex is what makes the enqueue and drain sides safe on different tasks, and it comes from the
RTOS — which is what brings the FreeRtos platform into the build. A single-task device injects
SolidSyslogNullMutex_Get() instead and pays nothing.

SolidSyslog_Service returns a status a device wanting more sophisticated scheduling can drive
from. A loop with a delay is the simplest model that works.

When you need it. Once logging and sending are decoupled, the buffer has to absorb however many
events can be logged before it is next serviced. It also makes logging from multiple tasks safe.

[DavidCozens]

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