Gossip Convergence Benchmark

[cb7chaitanya]

Measure convergence across:

• cluster size
• fanout
• message size

Produce graphs.

[anmol0b]

starting this one

[anmol0b]

@cb7chaitanya, resolved this in PR #51 instead of a separate PR.

Had two options for the output:

• PNG charts via plotters : charts committed directly to the repo, no extra tooling
• CSV + interactive HTML page : same Rust binary, browser renders the charts

Went with plotters. Let me know if you'd prefer the HTML approach!

i'm underconfident on this one