Fix FPS reporting

[m-RNA]

Fix: #24

Summary:

• Fix FPS computation so 100-frame windows no longer truncate to 0 fps; use math with divide-by-zero guard.
• Correct initial timestamp handling to avoid incorrect timing on the first logged window.
• Optimize logging by only doing the 100-frame timing/size calculations when the log level is INFO, minimizing overhead when info logs are disabled.

Before:

I (1863) main_task: Returned from app_main()
I (3863) [STREAM_SERVER]: Size: 5KB, Time: 8341ms (0fps)
I (6109) [STREAM_SERVER]: Size: 5KB, Time: 2245ms (0fps)
I (8360) [STREAM_SERVER]: Size: 5KB, Time: 2251ms (0fps)

After:

I (6330) main_task: Returned from app_main()
I (9513) [STREAM_SERVER]: 100 Frames Size: 4KB, Time: 2250ms (44fps)
I (11788) [STREAM_SERVER]: 100 Frames Size: 4KB, Time: 2275ms (43fps)
I (14031) [STREAM_SERVER]: 100 Frames Size: 4KB, Time: 2244ms (44fps)