feat(cli): organize help flags into logical sections

[0xWeakSheep]

Motivation

#408.

The contender spam --help output had grown large and hard to scan quickly.
This change groups flags by logical intent and improves readability for high-frequency commands.

Solution

• Added explicit help section headings for CLI arguments:
  • Common
  • Payload Adjustments
  • Runtime Adjustments
  • Engine API Settings
• Applied grouped help headings to shared CLI arg structs so the same organization is reused across commands.
• Updated spam-specific and campaign-specific flags to align with the same section layout.
• Added changelog entries in root and crates/cli changelogs.

Scope

Primary focus:

• contender spam --help

Also aligned:

• contender campaign --help

Validation

• cargo +nightly fmt --all
• cargo check -p contender_cli
• cargo run -p contender_cli --bin contender -- spam --help
• cargo run -p contender_cli --bin contender -- campaign --help

Notes

This is a help/UX-only change. No runtime behavior was modified.