chore: Java 25 compile+runtime baseline, CI matrix 25/26 (plan-01 A1)

[cuioss-oliver]

Post-merge amendment A1 to Plan 01: drop the "compile at 21, run at 25" split in favour of compiling and running on Java 25 (the LTS). Java 21 is not a target for this project.

Changes

• pom.xml: maven.compiler.release and the compiler-plugin <release> 21 → 25.
• .github/project.yml: maven-build.java-versions ["21","25"] → ["25","26"]; primary java-version 21 → 25. Add "27" at its GA (~Sept 2026).
• doc/adr/0001-*: rewritten (status accepted; supersedes the proposed compile-low/run-high form).
• doc/plan/01-*: new Post-Merge Amendments → A1 section recording this change (the plan already merged, so it is amended, not rewritten).
• CLAUDE.md: project-overview + language-features lines updated to Java 25.

Verification

Reactor compiles clean at --release 25 (bytecode major version 69 = Java 25) on a JDK 26 toolchain. The JDK-26 CI matrix leg is a forward-compatibility guard (the Lombok/Unsafe runtime warning under 26 is expected and non-blocking).

Notes

• Native Mandrel builder pin stays on the jdk-25 line (already 25 — unchanged).
• Descriptive Java-version prose in architecture.adoc / README.adoc / plan-02 is updated in the sibling docs PR (docs/address-design-findings); the two PRs touch disjoint files.

Summary by CodeRabbit

• New Features

  • Updated the project’s supported Java baseline to Java 25 for compilation, runtime use, and native builds.
  • Added Java 26 CI coverage for forward compatibility.

• Documentation

  • Updated project guidance and architecture documentation to reflect the Java 25 requirement, supported language features, build environments, and upgrade strategy.
  • Clarified post-merge implementation decisions and current deployment expectations.

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Review details

⚙️ Run configuration

Configuration used: Repository: cuioss/coderabbit/.coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 72e58310-a99a-45e7-a781-f1db094a7cad

📥 Commits

Reviewing files that changed from the base of the PR and between 577d2e7 and bbb35b2.

📒 Files selected for processing (1)

• pom.xml

📝 Walkthrough

Walkthrough

The project now compiles and runs on Java 25, tests Java 25 and 26, and documents the accepted baseline across its ADR, implementation plan, and development guidance.

Changes

Java 25 compile and runtime baseline

Layer / File(s)	Summary
Baseline decision and policy doc/adr/0001-java25-runtime-baseline.adoc	The ADR is accepted with Java 25 as the compilation and runtime baseline, Java 22–25 language features enabled, Java 25 native/JVM shipping paths, and Java 26 as forward-compatibility coverage.
Build and CI configuration pom.xml, .github/project.yml	Maven compiler release settings move from 21 to 25, and the CI Maven matrix changes from 21/25 to 25/26 with Java 25 as the primary version.
Project guidance updates CLAUDE.md, doc/plan/01-base-implementation.adoc	Project guidance and the implementation plan record Java 25 compile/runtime usage, Java 25 feature guidance, and post-merge baseline amendments.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: moving the project to a Java 25 compile/runtime baseline with a 25/26 CI matrix.

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[gemini-code-assist]

Code Review

This pull request updates the project's compile and runtime baseline to Java 25, aligning the build configuration, CI matrix, and documentation (including ADR-0001 and Plan 01 amendments) to target Java 25 and 26. The feedback suggests using the ${maven.compiler.release} property in the maven-compiler-plugin configuration within pom.xml to avoid duplicating the version number.

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