feat: spool records to a file store with a CRC-16 at rest

[DavidCozens]

Spool to a SolidSyslogBlockStore over a SolidSyslogFileBlockDevice over the library's FatFs
port, replacing the Null store. The service task drains the ring into storage and sends from there,
so a failed send costs a retry rather than the record: the audit trail survives an outage instead of
ending at it.

#define SYSLOG_STORE_PREFIX "syslog"
#define SYSLOG_STORE_BLOCKS 4U

struct SolidSyslogBlockStoreConfig storeConfig = {
    .BlockDevice    = SolidSyslogFileBlockDevice_Create(SolidSyslogFatFsFile_Create(), SYSLOG_STORE_PREFIX, 0U),
    .MaxBlocks      = SYSLOG_STORE_BLOCKS,
    .DiscardPolicy  = SOLIDSYSLOG_DISCARD_POLICY_OLDEST,
    .SecurityPolicy = SolidSyslogCrc16Policy_Create(),
};

Three decisions come with it: how much to store, which is capacity on the medium rather than RAM;
what happens when it fills — discard oldest, discard newest, or halt; and whether to be warned
before that point, via the capacity-threshold callback.

This device stores four blocks, one file per block, syslog00.log upward on the volume it already
mounts, and discards the oldest when full.

The CRC-16 detects corruption, not tampering. It catches a truncated write or bit-rot; anyone who
can edit a stored record can recompute it. It establishes that a record came back the way it went
in, which is the prerequisite for spooling at all. Making stored records tamper-evident, and then
unreadable, are later stages.

Storing happens on the service task, so a task that calls SolidSyslog_Log still knows nothing
about what happens after it returns and its stack does not move. The RAM is pool allocation and
handles rather than buffers — nothing holds a block in memory, so the store costs its handles
rather than its capacity.

FatFs joins SOLIDSYSLOG_PLATFORMS, and its sources compile against this device's own ffconf.h
like the rest of the application.

When you need it. If losing the records raised during an outage is not acceptable, or if they
must survive a reboot.

[DavidCozens]

@coderabbitai pause

[coderabbitai]

Note

Reviews paused

Use the following commands to manage reviews:

• @coderabbitai resume to resume automatic reviews.
• @coderabbitai review to trigger a single review.

Use the checkboxes below for quick actions:

• ▶️ Resume reviews
• 🔍 Trigger review

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d2df1355-1ec6-4e61-94fa-9b8dac0b65da

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

✅ Action performed

Reviews paused.