Allow reverting mob head progression in the Purifier

[rubensworks]

Closes #1258

Adds a way to reverse the Blood Infuser mob head progression, using the Purifier as suggested in the issue.

Behaviour

The Blood Infuser chain is skeleton_skull → (1000 mB) → zombie_head → (5000 mB) → creeper_head → (25000 mB) → wither_skeleton_skull. The Purifier now walks it back one step at a time:

• Accepts Wither Skeleton Skull, Creeper Head and Zombie Head in the main slot.
• Requires a full tank (3 buckets of blood) and consumes all of it, taking 60 ticks, matching the existing disenchant action.
• One step per cycle, so wither skull → skeleton skull takes three full refills. Net lossy against the infuser costs, which seemed right for a revert.
• No additional-slot item needed.

Implementation notes

The Purifier is not data-driven; it runs a Java registry of IPurifierAction implementations, so this is a new MobHeadDowngradePurifyAction rather than a recipe file. It is registered last, so an enchanted head plus a Blook still disenchants first.

A tag-based input check was considered and rejected: 1.20.1 has no vanilla skull/head item tag, and while Forge provides forge:heads, a tag only gives membership, not the mapping of which head reverts to which. It would also wrongly advertise dragon heads, player heads and skeleton skulls as valid Purifier inputs.

Also adds an info book paragraph (purifier.text4) documenting the new behaviour.

Testing

./gradlew compileJava passes.

No tests here: this branch has no src/test/java, and being 1.20.1 there is no gametest infrastructure either. Game tests covering this live in #1259 against master-1.21-lts, ready for the upmerge. They were verified there against a temporary port of this implementation: all 81 game tests pass with it, and testPurifierMobHeadDowngrade fails without it.

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https://claude.ai/code/session_01DwM8cKrtoseKe4UBtH6NLL

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