S04.05: Circular buffer — bare-metal ring buffer with mutex injection

[DavidCozens]

Parent epic: #6

Portable in-memory ring buffer satisfying the SolidSyslogBuffer interface, suitable for bare-metal / RTOS targets without POSIX message queues, and as the buffer of choice for Windows.

Design

• Storage: caller-supplied, Formatter-style variable-size pattern. Two sizing macros: SOLIDSYSLOG_CIRCULARBUFFER_STORAGE_SIZE(maxMessages) for the friendly "N max-sized messages" form, and SOLIDSYSLOG_CIRCULARBUFFER_STORAGE_SIZE_BYTES(ringBytes) for raw byte capacity (useful in tests). _Create takes storage, sizeof(storage), and the mutex. No internal malloc.
• Record framing: each record is a uint16_t size header followed by size payload bytes. Records do not span the wrap point — wrap is handled by wrapPoint index and a no-split rule, not by record splitting. Payloads exceeding SOLIDSYSLOG_MAX_MESSAGE_SIZE are dropped at Write.
• Overflow policy: drop-newest, silently. Matches the existing PosixMessageQueueBuffer (O_NONBLOCK + ignored mq_send return). Loss is detected SIEM-side by gaps in origin sequenceId (the mechanism that closed S04.06).
• Read semantics: if the next record does not fit in the caller-supplied destination buffer, Read returns false with bytesRead = 0 and leaves the record queued — mirrors mq_receive EMSGSIZE semantics.
• Concurrency: synchronization via injected SolidSyslogMutex vtable (Lock/Unlock). SolidSyslogNullMutex is the no-op default for single-task use.

Deliverables in this story

• SolidSyslogCircularBuffer + tests (main behaviour, mutex call-pattern, small-ring boundary tests)
• SolidSyslogMutex vtable + SolidSyslogNullMutex + tests
• SolidSyslogPosixMutex (pthread_mutex_t) + tests
• SolidSyslogWindowsMutex (CRITICAL_SECTION) + tests
• Public-header audience table updated in CLAUDE.md

Out of scope

• Wiring this buffer through any example or BDD harness — that is S13.18 (S13.18: Windows BDD exercising the portable ring buffer #244).
• Encryption / authentication of records (might revisit if this is ever used cross-process; not today).
• Configurable discard policy. If we ever need drop-oldest / halt at this layer, reuse SolidSyslogDiscardPolicy from BlockStore.
• Mutex _Create initialization-failure handling (pthread_mutex_init / InitializeCriticalSection return values) — deferred to E12 (error handling), tracked in the project robustness backlog.