docs(deploy_gate): correct the rule-5 companion-test count

[argszero]

Summary

Rule 5 of the deploy gate (the quick-start must hand the master key to compose) has four companion tests in src/deploy_gate.rs, but the rule's module note says three. This corrects the count and names the fourth role.

Rule 4 genuinely has three, and its note carries the same sentence word for word — so this line was copied from there and the count never moved with it. The count was wrong when it was written: 8cb559a8 (#338) added all four tests and this sentence in the same commit.

The fourth test is the arming guard (the_quickstart_rule_is_armed_on_the_live_compose). It asserts that compose still marks ATP_MASTER_KEY as required, which is what keeps the axis test from passing vacuously on a rule whose trigger has been switched off.

Related Issue

(none — found by this repository's own comment-claim review)

Changes

• src/deploy_gate.rs: the rule-5 note now reads "four tests (axis + positive control + rule tooth + arming guard)", and the line is re-wrapped to stay inside the file's comment width
• no configuration or data-structure change

Tests

• cargo test — 562 passed, 0 failed
• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• new tests: not applicable — the change is a comment inside a #[cfg(test)]-only module (src/main.rs:48-49), so no behaviour is touched

Checklist

• branch name follows the convention (docs/…)
• commit message uses Conventional Commits
• single responsibility, minimal change (1 file, +2/−1)