fix: safety hardening for pointer casts, integer bounds, and magic constants

[kacy]

Summary

• add upper bound check to inotify event offset validation in dev watcher processEvents, matching the existing check in processEventsMulti
• add explicit maxInt upper bound checks before @intcast for cpu_cores (u32), memory_mb (u64), gpu_count (u32), gpu_vram_mb (u64) in agent registration
• extract magic buffer size 16 * 1024 to named constant stream_buffer_size in http2 passthrough