Add engine-combat branch coverage tests

35 new tests covering combat phase validation edge cases:
beginCombatPhase/processCombat/skipCombat phase and player guards,
combatDisabled rejection, duplicate/invalid attacker checks, empty
attackers, landed target, split-fire hex validation, attack strength
limits, group type mismatch, ordnance targeting rules (friendly,
non-nuke, destroyed, reduced-strength), anti-nuke hit/miss resolution,
shouldEnterCombatPhase branches (hazards, disabled ships, ordnance
targets), and skip combat with base defense.

Branch coverage: 70.2% -> 88.4%

Author: rgilks