build: upgrade Gradle 9.1.0 + AGP 9.1.0 + Dokka 2.1.0 foundation

[kirich1409]

Summary

Foundation task for the AGP 9 migration. Bumps version numbers and adds the new KMP library plugin alias so parallel migration tasks can branch off this.

Changes

• gradle/wrapper/gradle-wrapper.properties: 8.14.3 → 9.1.0
• gradle/libs.versions.toml: agp → 9.1.0, dokka → 2.1.0, add androidKmpLibrary alias
• build.gradle.kts (root): register androidKmpLibrary apply false
• gradle.properties: add org.gradle.parallel=true, add temporary android.enableLegacyVariantApi=true

Instructions for subagent

Follow Task 1 steps in docs/superpowers/plans/2026-03-22-agp9-migration.md exactly.
Temporary flag android.enableLegacyVariantApi=true is intentional — removed in Task 6.
Do NOT set android.builtInKotlin=false.