Modernize BiomeTunes as a Fabric 26.2 monorepo

[hampfh]

Summary

• convert the repository into an apps-based monorepo while preserving the archival datapack unchanged
• add a client-only Kotlin/Fabric mod for Minecraft 26.2 with the canonical 15-track soundpack bundled into one installable JAR
• add validated 66-biome mappings, boss fallback, equal-power crossfades, configuration persistence, live audio lifecycle handling, settings UI, client-only /biometunes command, and optional Mod Menu support
• add standalone soundpack packaging, CI, fail-closed release automation, documentation, credits, and manual verification gates
• verify complete release archives, including metadata, exact resource inventory, duplicate detection, source-audio isolation, and byte-identical audio

Verification

• ./gradlew clean build packageSoundpack verifyDistributionArchives
• 73 tests, 0 failures, 0 errors, 0 skipped
• committed negative archive contract rejects metadata, mixin/icon, and source-OGG corruption
• Minecraft 26.2 development client reached stable startup both with and without Mod Menu
• final senior review: 0 Critical and 0 Important findings open
• no .superpowers or .context paths exist in reachable branch history

Release gates

This PR prepares publication but does not publish anything. Before tagging v1.0.0, complete all 17 manual audio/gameplay checks, confirm the legacy forest track attribution wording, configure Modrinth as client required/server unsupported, provide publication credentials, and set RELEASE_APPROVED_TAG only after explicit release authorization.