ref(logs): extract reusable batcher for metrics

[jpnurmi]

Summary

• Extracts the batching infrastructure from sentry_logs.c into a reusable sentry_batcher module
• Prepares for metrics support by making the batcher telemetry-type agnostic

Changes

New files:

• src/sentry_batcher.h - Batcher types and API declarations
• src/sentry_batcher.c - Batcher implementation

Key features of the batcher:

• Lock-free double-buffer pattern for high-throughput producer threads
• Configurable batch_func callback for telemetry-specific envelope creation
• Thread lifecycle management (STOPPED → STARTING → RUNNING)
• Crash-safe flush mode for signal handler contexts
• Test helper sentry__batcher_wait_for_thread_startup() for avoiding race conditions

API:

void sentry__batcher_startup(sentry_batcher_t *batcher, sentry_batch_func_t batch_func);
bool sentry__batcher_enqueue(sentry_batcher_t *batcher, sentry_value_t item);
void sentry__batcher_shutdown(sentry_batcher_t *batcher, uint64_t timeout);
void sentry__batcher_flush(sentry_batcher_t *batcher, bool crash_safe);
void sentry__batcher_force_flush(sentry_batcher_t *batcher);
void sentry__batcher_flush_crash_safe(sentry_batcher_t *batcher);

Usage example (from sentry_logs.c):

static sentry_batcher_t g_batcher = { ... };

void sentry__logs_startup(void) {
    sentry__batcher_startup(&g_batcher, sentry__envelope_add_logs);
}

Test plan

• Existing logs tests continue to pass
• Build succeeds on all platforms

Relates to #1453

[JoshuaMoelans]

Nice, LGTM!