build: pin the Gradle daemon JVM to 21

[jamesarich]

Summary

Adds gradle/gradle-daemon-jvm.properties with toolchainVersion=21, so the Gradle daemon runs on Java 21 whatever JDK the launcher or the CI runner offers. Fixes :detekt on the JDK 25 runners that #81 introduced, and unblocks #82.

Type of change

• Infrastructure / CI / build

Related issue / discussion

Follows #81 (java-jdk v25). Unblocks #82. Supersedes #83, which reverted #81 instead — this is the better shape, because it fixes the class of problem rather than one instance.

detekt 1.23.8 invokes its analysis in-process on the Gradle daemon's JVM, and the embedded Kotlin compiler's shaded IntelliJ JavaVersion.parse rejects a feature version of 25:

Caused by: java.lang.IllegalArgumentException: 25.0.4.1
  at org.jetbrains.kotlin.com.intellij.util.lang.JavaVersion.parse(JavaVersion.java:307)
  at org.jetbrains.kotlin.com.intellij.util.lang.JavaVersion.current(JavaVersion.java:176)
  at org.jetbrains.kotlin.cli.jvm.modules.JavaVersionUtilsKt.isAtLeastJava9(javaVersionUtils.kt:11)
  at org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment.<init>(KotlinCoreEnvironment.kt:243)
  at io.github.detekt.parser.KotlinEnvironmentUtilsKt.createKotlinCoreEnvironment(KotlinEnvironmentUtils.kt:61)
  at io.gitlab.arturbosch.detekt.core.tooling.AnalysisFacade.runAnalysis

1.23.8 is the newest stable detekt, so there is nothing to bump to.

How was this verified?

Measured both ways on this machine, against a real JDK 25 requested explicitly, with --rerun-tasks --no-build-cache so nothing could be replayed:

	./gradlew detekt --rerun-tasks --no-build-cache -Dorg.gradle.java.home=<JDK 25>
without this file	FAILED — Execution failed for task ':detekt' / > 25.0.3
with this file	BUILD SUCCESSFUL, 1 actionable task: 1 executed

Also confirmed jvmToolchain(21) was never the issue: :jvmMainClasses --rerun-tasks --no-build-cache with the daemon on JDK 25 still emits major-version 65 (Java 21) bytecode, per javap -v. #81 was a CI-runner change, not a published-bytecode change.

Why #81's CI did not catch this

:detekt was served FROM-CACHE on #81's PR run and on its merge-queue run — nothing detekt reads had changed, so the new runner JDK was never exercised. The first build that actually re-ran detekt under JDK 25 was the merge-queue run for #82, which failed. On this PR detekt will very likely be FROM-CACHE again for the same reason; #82's merge-queue run is the build that genuinely proves the fix, because the Kotlin bump invalidates detekt's inputs.

Affirmations

• All commits are signed off (DCO).
• I have read CONTRIBUTING.md.
• No public API change.
• No codec or wire-frame change.
• ./gradlew build passes locally.

Notes for reviewers

• No toolchainUrl.* download entries. ./gradlew updateDaemonJvm refuses to generate them here — Toolchain download repositories have not been configured — because this build deliberately carries no foojay resolver. Without them Gradle must detect a JDK 21 rather than download one; GitHub's runner images ship one, and CONTRIBUTING.md already requires JDK 21 locally. Adding the foojay resolver to settings.gradle.kts is the follow-up if that ever bites.
• This brings kzstd in line with the org's other KMP repos. MQTTastic-Client-KMP, meshtastic-sdk and meshtastic-node-kmp already carry this file; kzstd, TAKPacket-SDK and gradle-flatpak-sources did not, and those last two run detekt 1.23.x too — they have the same latent break the moment they take a runner-JDK bump.

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📥 Commits

Reviewing files that changed from the base of the PR and between 54ccef4 and a2687be.

📒 Files selected for processing (1)

• gradle/gradle-daemon-jvm.properties

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