Fix #247: fix gun firepower cost scaling for live ROF limits

[Ally-for-Hire]

Addresses #247

Gun firepower cost now responds to live ROFLimit changes instead of staying pinned to the static spawn-time ACEPoints value. This makes lowering a gun's RPM reduce its firepower cost as expected.

What Changed

• Firepower subsystem scoring for acf_gun now uses a runtime gun cost helper instead of the static entity points value.
• Added a live gun RPS helper that computes configured sustained RPS from the gun's current setup and optional ROF limit.
• Gun firepower cost now scales by current ROF threat relative to the gun's unthrottled base ROF threat.
• Changing ROFLimit now marks both Firepower and Ammo subsystem caches dirty immediately so contraption rescans do not keep stale totals.

Notes

• This is based on live configuration, not transient firing state.
• It intentionally does not make point cost fluctuate with temporary loader stamina changes during combat.