Hedgehog explosion immunity rebalance (#6104)

# About the pull request
Hedgehog now only has explosion stun immunity if you're above half shard
rate.

# Explain why it's good for the game

Hedgehog is arguably one of the safest castes right now, always has
grenade immunity, no longer has a slowdown and an increasing armor rate
with its spikes and an invincibility button.

The immunity was given due to the slowdown it naturally had, but recent
changes have touched that without removing or touching up its immunity,
this tries to fix that.


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
balance: Hedgehog now gains explosion immunity to nades if you have
above half shard amount.
/:cl:

---------

Co-authored-by: InsaneRed <[email protected]>
Co-authored-by: Birdtalon <[email protected]>
Co-authored-by: Drathek <[email protected]>

code/modules/mob/living/carbon/xenomorph/strains/castes/ravager/hedgehog.dm

@@ -1,6 +1,6 @@
 /datum/xeno_strain/hedgehog
 	name = RAVAGER_HEDGEHOG
-	description = "You lose your empower, charge, scissor cut and some slash damage, for a bit more explosive resistance, immunity to small explosions, and you gain several new abilities that allow you to become a spiky tank. You build up shards internally over time and also when taking damage that increase your armor's resilience. You can use these shards to power three new abilities: Spike Shield, which gives you a temporary shield that spits bone shards around you when damaged, Fire Spikes, which launches spikes at your target that slows them and does extra damage if they move, and finally, Spike Shed, which launches spikes all around yourself and gives you a temporary speed boost as an escape plan at the cost of all your stored shards and being unable to gain shards for thirty seconds."
+	description = "You lose your empower, charge, scissor cut, and some slash damage in exchange for more explosive resistance. Your resistance scales with your shard count and at 50% grants you immunity to some explosive stuns. You accumulate shards over time and when taking damage. You can use these shards to power three new abilities: Spike Shield which gives you a temporary shield that spits bone shards around you when damaged; Fire Spikes which launches spikes at your target to slow them and deal damage when they move; and Spike Shed which launches all your spikes, grants a temporary speed boost, and disables shard generation for thirty seconds."
 	flavor_description = "They will be of iron will and steely muscle. In great armor shall they be clad, and with the mightiest spikes will they be armed."
 	icon_state_prefix = "Hedgehog"
 
@@ -19,7 +19,7 @@
 
 /datum/xeno_strain/hedgehog/apply_strain(mob/living/carbon/xenomorph/ravager/ravager)
 	ravager.plasma_max = 0
-	ravager.small_explosives_stun = FALSE
+	ravager.small_explosives_stun = TRUE
 	ravager.explosivearmor_modifier += XENO_EXPOSIVEARMOR_MOD_SMALL
 	ravager.damage_modifier -= XENO_DAMAGE_MOD_SMALL
 
@@ -72,7 +72,6 @@
 
 	bound_xeno.speed_modifier += shard_lock_speed_mod
 	bound_xeno.recalculate_speed()
-
 	shards_locked = FALSE
 
 // Return true if we have enough shards, false otherwise
@@ -103,6 +102,15 @@
 	var/percentage_shards = round((shards / max_shards) * 100, 10)
 	if(percentage_shards)
 		holder.overlays += image('icons/mob/hud/hud.dmi', "xenoenergy[percentage_shards]")
+
+	if(percentage_shards >= 50)
+		bound_xeno.small_explosives_stun = FALSE
+		bound_xeno.add_filter("hedge_unstunnable", 1, list("type" = "outline", "color" = "#421313", "size" = 1))
+	else
+		bound_xeno.small_explosives_stun = TRUE
+		bound_xeno.remove_filter("hedge_unstunnable", 1, list("type" = "outline", "color" = "#421313", "size" = 1))
+
+
 	return