Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@lib/kv-router/benches/mooncake_bench.rs`:
- Around line 1168-1171: Replace fragile substring replacements on
args.sweep_output with path-based extension replacement: use
std::path::Path::with_extension (e.g.
Path::new(&args.sweep_output).with_extension("json").to_string_lossy().into_owned())
to build json_path from args.sweep_output instead of .replace(".png", ...) /
.replace(".svg", ...); apply the same change at the other occurrence mentioned
(around line with the second replacement).
- Around line 1079-1084: The sweep generation code uses args.sweep_steps as n
and divides by (n - 1), which is invalid when sweep_steps <= 1; update the logic
around n and the durations_low_to_high computation to guard against degenerate
inputs by checking args.sweep_steps (or n) first: if n <= 1 produce a sensible
single-entry durations vector (e.g., one value derived from log_min/log_max or
return an error/early exit), otherwise perform the existing interpolation using
(n - 1); ensure this change touches the block that computes
durations_low_to_high and any callers that expect multiple steps so downstream
throughput math/plotting remains stable.