Add Minecraft 26.2 support

[chorusfruit-233]

Summary

Adds support for Minecraft 26.2 while keeping the existing v26_R1 implementation.

Changes

• Map Minecraft 26.2, including Paper versions ending in .build, to v26_R1
• Update the Spigot dependencies to 26.2-R0.1-SNAPSHOT
• Replace the removed EntityType.ITEM_FRAME reference with an ItemFrame constructor available in both 26.1.x and 26.2

No project version bump, public API changes, dependency upgrades, or unrelated refactoring are included.

Testing

• Built Spigot 26.2 using BuildTools and JDK 25
• Compiled the updated 26.x implementation against Spigot 26.2
• Verified the generated bytecode uses the ItemFrame constructor shared by 26.1.2 and 26.2
• Started a Spigot 26.2 server and confirmed the plugin loads, enables, and disables without errors
• Ran git diff --check

The full historical Maven reactor was not completed because the existing ProtocolLib 5.1.0 dependency could not be resolved from the configured repositories. No build configuration was changed to work around this.

AI Usage Disclosure

This pull request was produced entirely using OpenAI Codex. Repository analysis, implementation, compatibility investigation, compilation, and the Spigot startup smoke test were all performed by the AI. No source changes were manually authored.