feat: per-module CHANGELOG.md support

[AnteLund]

Summary

Adds a new changelog: "module" option to .changeset/config.json so that, with independent versioning, each bumped submodule gets its own CHANGELOG.md next to its pom.xml instead of one aggregated file at the reactor root.

• New ChangelogMode.MODULE enum value on ChangesetsConfig, with validation that rejects it when combined with fixed versioning (all modules would share one version anyway).

• PrepareMojo.writeChangelog branches on the mode: in MODULE mode it writes one file per bumped module.

• When a BOM is also configured, the reactor-root CHANGELOG.md is additionally written as a dependabot-style rollup — one nested entry per bumped module under a single consumer-parent@<bomVersion> header — so consumers still get an at-a-glance overview of what moved in that BOM release while the authoritative per-module history lives next to each module.

• Behavior is unchanged when changelog is absent (defaults to root); existing repos need no config edit.

• Documentation

• Unit- and Integration test

[Copilot]

Pull request overview

Adds support for per-module changelog generation in independent-versioned multi-module Maven projects by introducing a new changelog: "module" configuration mode, with BOM-aware rollup behavior preserved via an additional root changelog when applicable.

Changes:

• Introduces ChangesetsConfig.ChangelogMode.MODULE and validates it against fixed versioning.
• Updates PrepareMojo to write one CHANGELOG.md per bumped module in MODULE mode, plus an optional BOM rollup at the reactor root.
• Adds documentation and new integration tests covering module and module+ BOM scenarios.

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents changelog: "module" behavior and configuration reference updates.
changesets-maven-plugin/src/main/java/se/fortnox/changesets/maven/PrepareMojo.java	Branches changelog writing to per-module mode; keeps BOM rollup behavior.
changesets-maven-plugin/src/it/module-changelog/verify.groovy	IT verification for per-module changelog output (no BOM).
changesets-maven-plugin/src/it/module-changelog/pom.xml	New IT project for module changelog scenario.
changesets-maven-plugin/src/it/module-changelog/module-a/pom.xml	IT module A setup.
changesets-maven-plugin/src/it/module-changelog/module-a/EXPECTED_CHANGELOG.md	Expected per-module changelog for module A.
changesets-maven-plugin/src/it/module-changelog/module-b/pom.xml	IT module B setup.
changesets-maven-plugin/src/it/module-changelog/module-b/EXPECTED_CHANGELOG.md	Expected per-module changelog for module B.
changesets-maven-plugin/src/it/module-changelog/invoker.properties	Maven Invoker configuration for the new IT.
changesets-maven-plugin/src/it/module-changelog/.changeset/config.json	IT config enabling independent + module changelog mode.
changesets-maven-plugin/src/it/module-changelog/.changeset/bump-a.md	IT changeset for module A bump.
changesets-maven-plugin/src/it/module-changelog/.changeset/bump-b.md	IT changeset for module B bump.
changesets-maven-plugin/src/it/module-changelog-bom/verify.groovy	IT verification for per-module changelog output with BOM rollup.
changesets-maven-plugin/src/it/module-changelog-bom/pom.xml	New IT project for module changelog + BOM scenario.
changesets-maven-plugin/src/it/module-changelog-bom/invoker.properties	Maven Invoker configuration for the BOM IT.
changesets-maven-plugin/src/it/module-changelog-bom/EXPECTED_CHANGELOG.md	Expected root rollup changelog for BOM scenario.
changesets-maven-plugin/src/it/module-changelog-bom/consumer-parent/pom.xml	IT consumer-parent module for BOM header behavior.
changesets-maven-plugin/src/it/module-changelog-bom/bom/pom.xml	IT BOM module with pinned properties for starters.
changesets-maven-plugin/src/it/module-changelog-bom/starter-a/pom.xml	IT starter A setup.
changesets-maven-plugin/src/it/module-changelog-bom/starter-a/EXPECTED_CHANGELOG.md	Expected per-module changelog for starter A.
changesets-maven-plugin/src/it/module-changelog-bom/starter-b/pom.xml	IT starter B setup.
changesets-maven-plugin/src/it/module-changelog-bom/starter-b/EXPECTED_CHANGELOG.md	Expected per-module changelog for starter B.
changesets-maven-plugin/src/it/module-changelog-bom/.changeset/config.json	IT config enabling BOM + per-module changelog mode.
changesets-maven-plugin/src/it/module-changelog-bom/.changeset/bump-a.md	IT changeset for starter A bump.
changesets-maven-plugin/src/it/module-changelog-bom/.changeset/bump-b.md	IT changeset for starter B bump.
changesets-java/src/test/java/se/fortnox/changesets/ChangesetsConfigTest.java	Unit tests for parsing/validating module changelog mode.
changesets-java/src/main/java/se/fortnox/changesets/ChangesetsConfig.java	Adds ChangelogMode.MODULE and validation against FIXED.
.changeset/swift-modules-changelog.md	Changeset documenting the feature addition.

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[magnusp]

@AnteLund should we apply the suggestion?

[AnteLund]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved the merge conflict in README.md (commit 27807c4). The conflict was in the ### Flow section — I kept the more detailed flow description from main (including the mvn changesets:prepare + mvn release:prepare release:perform snippet) while preserving the Configuration reference table added by this branch.

[magicus]

Suggested change

This header seems to have been mistakenly added.