Upgrade Kotlin to 2.4.20-Beta2

[vRallev]

Summary

• Upgrade the main build from Kotlin 2.4.10 to 2.4.20-Beta2; leave the standalone blueprints unchanged.
• Adapt the compiler plugin to the Kotlin 2.4.20 FIR, IR, and compiler-test APIs.
• Generate App Platform's renderer, robot, and scoped Metro binding containers in IR through Metro's extension API.
• Keep only cross-module contribution-discovery hints in FIR and explicitly run App Platform's declaration generation before Metro's IR graph transformation.
• Update compiler goldens and generated JVM API snapshots for the new IR output.

Issues found

Metro 1.4.0 enables IR class generation with Kotlin 2.4.20-Beta2. App Platform's former FIR declaration generators are therefore not invoked, which removes its generated bindings.

Moving the containers to IR uncovered two additional constraints:

1. Metro's FIR-hint option is coupled to IR class generation in its Gradle plugin, but Native and Wasm downstream compilations cannot discover package callables generated in IR. The build now leaves class generation in IR enabled while retaining FIR only for Metro's package-level contribution hints.
2. On Kotlin/Native, Metro must see App Platform's generated containers before it transforms and merges the graph. The Gradle plugins now set that compiler-plugin order explicitly.

The generated contribution containers are implementation details and no longer appear as source-level declarations or in JVM API snapshots.

Validation

• Full Android unit, iOS simulator, Desktop, Linux, and Wasm test matrix: 1,953 tasks passed.
• All 57 compiler-plugin tests passed.
• Gradle plugin tests passed.
• apiCheck, Detekt, Android lint, module-structure checks, generated test runners, and ktfmt passed.
• Fresh cross-module Wasm and iOS simulator compilations passed with Metro IR generation enabled.
• Configuration-cache storage and reuse passed.
• Production Wasm distributions for both sample and recipes passed.

Upstream references: Kotlin 2.4.20-Beta2, Kotlin EAP configuration, Metro compiler API, Metro issue #2576, and Metro PR #2590.