Fix a few combat and magic logic bugs

[Y0oshi]

• fellow_kt_killer check was inverted in OnDeath_HandleKillTask (introduced in extend summoning killtask multicredit bug to fellowships, capped with new server option #3561 when the logic was inlined from KillTask_GetEligibleReceivers). With the default (true), fellows got kill task credit even when the killer didn't have the quest, which is the opposite of the property description and the original logic. Note this changes live behavior on servers using the default.
• Exhaustion only triggered when a stamina cost landed exactly on 0. newVal is the unclamped value, so costs that overdrew stamina skipped OnExhausted. Checks the clamped value instead, which is how it worked before improved RunRate handling for players #2417.
• EnchantmentManager.Add didn't pass isWeaponSpell when writing a new layer, so weapon, cloak and aetheria procs that stacked onto an existing category picked up the Archmage's Endurance duration bonus. The first-layer and refresh paths already passed it.
• Invincible creatures only blocked melee, missile and hotspot damage. War/void projectiles, DoT ticks and direct-cast spells like Harm and Drain still affected non-player creatures (the DoT path even zeroed the notify amount but applied the damage anyway). Projectiles and DoTs now check target.Invincible, and TryResistSpell auto-resists for any invincible creature instead of only players.

[gmriggs]

Thanks for all the bug fixes @Y0oshi!