Add ProGuardFilesResourceTransformer to merge R8/ProGuard rule files

[Goooler]

Closes #1696.

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• CHANGELOG's "Unreleased" section has been updated, if applicable.

[Copilot]

Pull request overview

This PR adds first-class support in Shadow for merging dependency-provided R8/ProGuard rules shipped as resources, aligning with the feature request in #1696.

Changes:

• Introduces ProGuardFilesResourceTransformer to merge META-INF/proguard/** rule resources and relocate matched class/package tokens within rules.
• Adds unit + functional test coverage for matching, merging behavior, and relocation behavior.
• Updates documentation and public API surface to include the new transformer.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/ProGuardFilesResourceTransformer.kt	New transformer to merge and relocate ProGuard/R8 rule files under META-INF/proguard/**.
src/test/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/ProGuardFilesResourceTransformerTest.kt	Unit tests for default/custom path matching + merge/relocation behavior.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/ProGuardFilesResourceTransformerTest.kt	Functional coverage ensuring merged jar outputs contain expected entries and relocated rules.
src/test/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/DuplicatesStrategyCheckerTest.kt	Updates transformer counts/invocations to account for the new transformer.
docs/configuration/merging/README.md	Adds user-facing docs + transformer list/link for the new transformer.
docs/changes/README.md	Changelog entry for the new transformer feature.
api/shadow.api	Records the new public API class in the API dump.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

Suppressed comments (2)

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/ProGuardFilesResourceTransformer.kt:67

• CLASS_PATTERN will also match and relocate dotted filenames like com.foo.jar (e.g., in -injars / -libraryjars directives). Relocating those file paths would change the meaning of the ProGuard config. Consider skipping relocation for known file-path directives (or otherwise tightening the match so only class/package tokens are relocated).

    fun Iterable<Relocator>.relocateRuleLine(line: String): String {
      return when {
        line.isBlank() || line.trimStart().startsWith("#") -> line
        else ->
          CLASS_PATTERN.replace(line) { matchResult ->

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/transformers/ProGuardFilesResourceTransformer.kt:23

• The default include pattern is META-INF/proguard/**, which will also match non-rule resources (and potentially directories). Since this transformer is intended for rule files (issue scope calls out **.pro), consider restricting the default include to only *.pro files to avoid accidentally relocating/merging unrelated resources under that directory.

constructor(patternSet: PatternSet = PatternSet().include("META-INF/proguard/**")) :