Show max time per request in statistics

[fcsonline]

Summary

Adds a Max time per request line to both the per-task and global statistics blocks. Previously the worst-case figure shown was the 99.9th percentile, which (as noted in #188) often is not enough to understand a system's tail behavior. The max is read straight from the existing hdrhistogram via max(), so there is no extra bookkeeping or memory cost.

Example output

ping                      99.9'th percentile        477ms
ping                      Max time per request      477ms
...
99.9'th percentile        477ms
Max time per request      477ms

Notes

• cargo fmt --check, cargo clippy, and cargo build all pass.
• Verified at runtime that the line renders in both the per-task and global blocks.

Closes #188

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