Recurrent Complex Volts

Added missing-block repair tools for old .rcst structures.

• Missing blocks can now be replaced globally or fixed inside one editable structure.
• New GUI workflow: /rc gui -> Gen -> Fix.
• New commands: /rc missing ....

Improved compatibility for imported legacy structures and schematics.

• More old vanilla blocks now convert correctly to modern 1.20.1 blocks.
• Old schematics now use the same legacy block conversion path as RC structure files.

Improved stability for large old structure packs.

• Deferred placement is safer around rail/redstone-like blocks.
• Reduced repeated log spam from missing content and malformed legacy inventory data.