Lock the JVM-args contract of the Compiler launch tasks

[alexander-yevsyukov]

What

Turns the JVM fork options of the launch*SpineCompiler tasks into a supported,
regression-tested contract, so consumer builds can attach the JaCoCo agent to the forked
Compiler JVM and capture the coverage of the Compiler plugins executed there.

Implements Phases 0–1 of compiler-coverage-plan.md:

• LaunchTaskJvmArgsSpec (gradle-plugin/src/test) — in-process lock: the launch task is a
  plain JavaExec; consumer-added JVM args coexist with the task's own --add-opens defaults.
• LaunchTaskCoverageSpec + the coverage-agent-test fixture
  (gradle-plugin/src/functionalTest) — end-to-end lock: a real JaCoCo agent attached via
  jvmArgumentProviders reaches the fork, records plugin-class execution whose class IDs match
  the very jars the fork loaded (JaCoCo Analyzer reports covered lines > 0 on
  UnderscorePrefixRenderer), and an UP-TO-DATE launch keeps the .exec intact.
  The fixture doubles as the reference wiring consumers will copy.
• LaunchSpineCompiler KDoc now declares the fork options part of the task's public contract.
• RunnerTuning.tuneRunner() deduplicates the TestKit runner setup shared with PluginSpec.

Why

Code running inside the forked compiler JVM is invisible to JaCoCo/Kover, which instrument only
the test JVM — in validation, the two codegen modules sit at ~0% coverage despite being
exercised on every build. Phase 0 (documented in the plan) confirmed the fix needs no new API:
the fork options already survive, the fat CLI (compiler-cli-all) relocates nothing, and consumer
plugin classes reach the fork as original artifacts, so JaCoCo class IDs match by construction.
This PR locks those guarantees so a compiler upgrade cannot silently break consumer coverage
wiring (Phase 2, validation repo).

Verification

• ./gradlew build dokkaGenerate — BUILD SUCCESSFUL (full check incl. the whole
  functionalTest suite, detekt, and Dokka link resolution).
• Pre-PR reviewers: spine-code-review APPROVE, kotlin-engineer APPROVE,
  review-docs APPROVE WITH CHANGES — no must-fix items.

Review follow-ups (non-blocking, surfaced by the reviewers)

• Add a fenced usage example to the new KDoc section of LaunchSpineCompiler
  (per documentation-tasks.md item 1; the wiring currently lives only in the fixture).
• KDoc heading level: ### → ## for repo consistency.
• Strengthen the up-to-date assertion in LaunchTaskCoverageSpec to compare .exec length,
  not just existence; add a comment on the internal DefaultGradleRunner cast in
  RunnerTuning.kt; harmonize the cross-suite phrasing in LaunchTaskJvmArgsSpec.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR formalizes and regression-tests the public contract that launch*SpineCompiler tasks remain plain JavaExec tasks whose consumer-supplied JVM fork options (e.g., -javaagent: via jvmArgumentProviders) are preserved, enabling JaCoCo/Kover to capture coverage from code executed inside the forked Compiler JVM.

Changes:

• Adds an in-process unit spec (LaunchTaskJvmArgsSpec) and an end-to-end functional spec (LaunchTaskCoverageSpec + coverage-agent-test fixture) to lock the JVM-args/JaCoCo-agent behavior.
• Documents the instrumentation/JVM-args contract in LaunchSpineCompiler KDoc and refactors shared TestKit runner configuration into RunnerTuning.tuneRunner().
• Updates the compiler version used by the build/test ecosystem.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
version.gradle.kts	Bumps the Compiler version used by the build/integration tests.
gradle-plugin/src/test/kotlin/io/spine/tools/compiler/gradle/plugin/LaunchTaskJvmArgsSpec.kt	New unit-level contract test: launch task is JavaExec and preserves consumer JVM args alongside defaults.
gradle-plugin/src/main/kotlin/io/spine/tools/compiler/gradle/plugin/LaunchSpineCompiler.kt	Documents JVM fork options / instrumentation as part of the task’s supported contract.
gradle-plugin/src/functionalTest/resources/coverage-agent-test/src/main/proto/test.proto	Minimal proto input for the new functional fixture project.
gradle-plugin/src/functionalTest/resources/coverage-agent-test/settings.gradle.kts	Fixture settings (plugin resolution via mavenLocal()).
gradle-plugin/src/functionalTest/resources/coverage-agent-test/build.gradle.kts	Fixture wiring that attaches JaCoCo agent via jvmArgumentProviders and records plugin classpath for analysis.
gradle-plugin/src/functionalTest/kotlin/io/spine/tools/compiler/gradle/plugin/RunnerTuning.kt	New shared helper to tune TestKit runner JVM args and environment.
gradle-plugin/src/functionalTest/kotlin/io/spine/tools/compiler/gradle/plugin/PluginSpec.kt	Uses tuneRunner() instead of duplicating TestKit runner setup inline.
gradle-plugin/src/functionalTest/kotlin/io/spine/tools/compiler/gradle/plugin/LaunchTaskCoverageSpec.kt	New functional spec verifying agent reaches the forked JVM and produces analyzable .exec data.
gradle-plugin/build.gradle.kts	Adds org.jacoco.core dependency for functional tests to analyze .exec output.
buildSrc/src/test/kotlin/io/spine/gradle/VersionGradleFileSpec.kt	Extends tests for version.gradle.kts parsing (config-managed; not reviewed here).
buildSrc/src/main/kotlin/kmp-module.gradle.kts	Adjusts jvmTest JUnit/logging configuration (config-managed; not reviewed here).
buildSrc/src/main/kotlin/io/spine/gradle/VersionGradleFile.kt	Enhances parsing to support extra.set(...) declarations (config-managed; not reviewed here).
buildSrc/src/main/kotlin/io/spine/dependency/local/Compiler.kt	Updates fallback compiler versions (config-managed; not reviewed here).
.agents/tasks/compiler-coverage-plan.md	Adds the coverage capture plan document (config-managed; not reviewed here).
.agents/memory/project/functional-test-fixtures.md	Adds internal guidance for authoring functional-test fixtures (config-managed; not reviewed here).
.agents/memory/MEMORY.md	Registers the new memory entry (config-managed; not reviewed here).

[Copilot]

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

[codecov]

Codecov Report

❌ Patch coverage is 86.79245% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.60%. Comparing base (55c0289) to head (f8b7f15).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #79      +/-   ##
============================================
+ Coverage     76.51%   76.60%   +0.09%     
- Complexity      681      687       +6     
============================================
  Files           203      205       +2     
  Lines          3972     4018      +46     
  Branches        396      400       +4     
============================================
+ Hits           3039     3078      +39     
- Misses          807      810       +3     
- Partials        126      130       +4     

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