fix: delegate null-cost research lore to addon unlock-instructions in ItemButton

[KevinWoodWL]

Problem

ItemButton.getItemProvider renders guide.button.item.not-researched for researches whose cost == null (custom unlock flow, no research points). That key is a YAML section in the lang file (it has no-research-cost, enough-points, not-enough-points children), not a string. RebarTranslator reads it via ConfigAdapter.STRING which calls toString() on the MemorySection, so the player sees:

MemorySection[path='guide.button.item.not-researched',root='YamlConfiguration']

This appears on item buttons in the main guide page, recipe detail pages, and vanilla crafting pages — anywhere ItemButton is used to display a locked item.

Fix

ResearchButton already handles cost == null correctly by delegating to <namespace>.researches.<key>.unlock-instructions, letting the addon describe its own unlock flow. This PR aligns ItemButton with the same behaviour.

// before
Component.translatable("rebar.guide.button.item.not-researched")

// after
Component.translatable(
    "${research.key.namespace}.researches.${research.key.key}.unlock-instructions"
)

Addons that use null-cost researches can now provide a meaningful lore line on item buttons (e.g. "Requires 80 Material discipline points — unlock via Science Interface") rather than seeing garbled text.

Reproduce

1. Register a Research with cost = null and add an item to its unlocks set.
2. Open the Rebar guide while the research is not yet unlocked.
3. Any ItemButton showing that item displays MemorySection[...] as the first lore line.