feat: number every record with a meta sequenceId

[DavidCozens]

Add the first structured-data element, SolidSyslogMetaSd, carrying a sequence number. Elements
are supplied to the logger as an array and read on every record, so they must outlive the call that
creates the logger.

static struct SolidSyslogStructuredData* sd[1];

struct SolidSyslogMetaSdConfig metaConfig = {
    .Counter = SolidSyslogStdAtomicCounter_Create(),
};
sd[0] = SolidSyslogMetaSd_Create(&metaConfig);

struct SolidSyslogConfig config = {
    /* ... */
    .Sd      = sd,
    .SdCount = 1U,
};

... BOOT [meta sequenceId="1"] device started

The sequence number is incremented once per record formatted, not once per record delivered. A
record that never arrives therefore leaves a gap in the sequence rather than no trace at all, which
is why it is worth adding before any buffering or storage that could drop one. Instrument first,
then introduce the failure mode.

Unlike a header field, an SD PARAM has no nil value: one that is unset is omitted entirely rather
than written as -.

The counter comes from SolidSyslogStdAtomicCounter. If your toolchain has no atomics, supply your
own to the contract SolidSyslogAtomicCounter_Increment states — and note that logging from more
than one task is what makes the atomic part of it necessary.

StdAtomic joins the platform list, and here that is all it is: naming it compiles its two sources
exactly as naming LwipRaw compiles its eleven.

When you need it. If anyone needs to know that records have gone missing.

[DavidCozens]

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