build: bump compileSdk to 37 (unblocks androidx-core 1.19.0+)

[thebehera]

Context

PR #115 (bring dependencies current) held androidx-core at 1.18.0. The reason: androidx-core 1.19.0 sets minCompileSdk=37, but both Android modules currently compile against compileSdk 36, so AGP's checkReleaseAarMetadata / checkDebugAndroidTestAarMetadata fail:

Dependency 'androidx.core:core:1.19.0' requires libraries and applications that depend on it to compile against version 37 or later of the Android APIs … is currently compiled against android-36.

Bumping compileSdk touches the API 37 surface (lint, manifest merger, possible runtime behavior), so it was deliberately kept out of the toolchain PR.

Scope

• build.gradle.kts (base module): compileSdk = 36 → 37
• socket-quic/build.gradle.kts: compileSdk = 36 → 37
• gradle/libs.versions.toml: androidxCore 1.18.0 → 1.19.0 (or latest)
• Verify AGP 9.2.1 supports compileSdk 37 and CI's setup-android installs the API 37 platform
• Re-run ktlintCheck, :checkReleaseAarMetadata, and the android-emulator job
• Decide whether targetSdk should also move (compileSdk can advance independently)

Validation

• ./gradlew :checkReleaseAarMetadata :socket-quic:checkReleaseAarMetadata
• Full android-emulator CI job (the one that surfaced the failure in build: bring dependencies current (Gradle 9, AGP 9, Kotlin 2.3.21, buffer 5.3.1, liburing 2.14) #115)

Notes

• compileSdk can be raised without changing targetSdk/minSdk (minSdk stays 21 base / 24 socket-quic).
• The weekly dependency agent (ci: weekly Claude dependency-update agent + Dependabot #114) intentionally leaves AGP/SDK majors to a build-validated PR — this is exactly that kind of change.