Add Heavy Water requirement for TA5 Fusion Reactor

[standmit]

This merge request adds Heavy Water as a new gameplay requirement for the TA5 Fusion Reactor.

The Fusion Reactor remains a powerful late-game energy source, but it now requires a continuous supply of Heavy Water while running. This prevents it from being a completely maintenance-free perpetual energy source, while still allowing players to automate its fuel supply chain.

Changes

• Added Heavy Water as a new liquid.
• Added Heavy Water barrel support for storage and transport.
• Added Heavy Water production through the TA4 Liquid Filter.
• Added the Isotope Membrane Filter as a new consumable component for Heavy Water production.
• Added the Spent Isotope Membrane Filter, produced when the membrane filter is exhausted.
• Added Heavy Water consumption to the TA5 Fusion Reactor Controller.
• The reactor now reports a missing Heavy Water condition when it cannot continue operating.
• Updated TA4 and TA5 documentation to describe the new production chain and reactor requirement.

Gameplay Motivation

Previously, once built, the TA5 Fusion Reactor behaved almost like a perpetual energy source: it provided a very large amount of power without any ongoing material cost.

With this change, the reactor still remains valuable and highly efficient, but it now requires periodic maintenance through its Heavy Water supply chain. The limiting factor is not energy, but the Isotope Membrane Filter consumed during Heavy Water production.

Players can still run Heavy Water production using energy from the reactor itself, but they need to maintain the production line and replenish the isotope membrane filters over time.

Heavy Water Production

Heavy Water is produced in the TA4 Liquid Filter from water or river water using Isotope Membrane Filters.

During operation, the Isotope Membrane Filter slowly degrades and is eventually converted into a Spent Isotope Membrane Filter. River water causes slower membrane degradation.

Fusion Reactor Changes

Heavy Water must be supplied directly to the Fusion Reactor Controller.

During operation, the controller gradually consumes Heavy Water. If the internal Heavy Water buffer is empty, the reactor cannot continue normal operation and reports the missing resource.

Compatibility

Existing Fusion Reactor setups will need to be extended with a Heavy Water supply line.

Existing TA4 Liquid Filter behavior for previous recipes is preserved by the recipe-based Liquid Filter system that has already been merged in #281

[joe7575]

Cool feature, but this is a hard breaking change for servers with existing TA5 Fusion Reactor setups — currently running reactors will stop the moment this hits master, leaving every dependent factory without power.

For that reason I can't accept this as written. Your need to add a techage_fusion_reactor_heavy_water setting (default false) so that existing installations keep running after an update. New servers / new reactors can then opt in via settings.

Ideally the setting also gates the liquid filter recipes (heavy water output) so a server that stays on the old behavior doesn't get the maintenance chain unlocked either.