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.

SolidSyslog::FatFs is a header-configured upstream, so it is both named in SOLIDSYSLOG_PLATFORMS
and linked.

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📝 Walkthrough

Walkthrough

The baseline application now enables FatFs and replaces the null syslog store with a four-block, CRC16-protected FatFs block store. The README and run report document the file-store configuration, resource costs, measurements, and self-check values.

Changes

File store integration

Layer / File(s)	Summary
FatFs block-store wiring app/syslog/Syslog.c, CMakeLists.txt	Syslog_Start configures persistent FatFs storage with CRC16 protection, four-block capacity, and oldest-record eviction. The build enables and links SolidSyslog::FatFs.
File-store documentation README.md	The README documents configuration, discard behavior, CRC16 limitations, service-task storage, FatFs integration, and resource costs.
File-store measurement report run-report.md	The report updates file-store measurements, size totals, syslog output, and self-check references.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to e6831

The change adds file-backed spooling with CRC-16 protection; the only remaining issue is that the README example omits definitions for two storage macros, so the example needs a small documentation fix but presents no merge-blocking product risk.

Possibly related PRs

• cososo-ltd/solid-syslog-example#17: Implements the same FatFs-backed block store and CRC16 persistence changes.
• cososo-ltd/solid-syslog-example#10: Provides the earlier UDP passthrough and null-store logger wiring extended by this change.
• cososo-ltd/solid-syslog-example#21: Later changes the block-store security policy from CRC16 to HMAC-SHA256.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description clearly explains the capability and design, but it omits the required headings and checklist evidence for measurements, README generation, and ./run.sh.	Add the required sections and confirm the measurements CSV, stages.tsv entry, regenerated README, and successful ./run.sh run.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: file-store spooling with CRC-16 protection.

✨ Finishing Touches

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• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch stage-10-file-store

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[DavidCozens]

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Actionable comments posted: 1

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Inline comments:
In `@README.md`:
- Around line 19-25: Update the README storage configuration snippet before
struct SolidSyslogBlockStoreConfig to define SYSLOG_STORE_PREFIX and
SYSLOG_STORE_BLOCKS with the matching values used by the application, so the
copyable example compiles without undeclared identifiers.

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📒 Files selected for processing (4)

• CMakeLists.txt
• README.md
• app/syslog/Syslog.c
• run-report.md

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