feat(gradle): release v1.6.0 — Gradle build file version bump

Add internal/gradle package with ReadVersion and WriteVersion for
build.gradle (Groovy DSL, single-quoted) and build.gradle.kts (Kotlin
DSL, double-quoted). Quote style is preserved on write. regexp.QuoteMeta
ensures version strings with dots or special characters are safe.

Config follows the established multi-value pattern:
- gradle.build_file: single path (opt-in, no default)
- gradle.build_files: list for multi-module projects (overrides build_file)
- --gradle flag overrides build_file and clears build_files

100% per-package statement coverage maintained across all 13 packages;
FuzzReadVersion and FuzzWriteVersion added per fuzzing guidelines.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -1,6 +1,6 @@
 # releaser
 
-![release](https://img.shields.io/badge/release-v1.5.1-blue.svg)
+![release](https://img.shields.io/badge/release-v1.6.0-blue.svg)
 
 A CI-friendly release automation tool for GitFlow workflows using Conventional Commits.
 
@@ -23,7 +23,7 @@ release/1.2 branch
 2. **Tag discovery** — finds the latest tag matching `major.minor.*` on the current branch
 3. **Commit analysis** — parses Conventional Commits between last tag and HEAD
 4. **Version bump** — increments patch (the minor is owned by the branch)
-5. **Release** — updates `pom.xml`, commits, tags, creates GitLab or GitHub release
+5. **Release** — updates `pom.xml` / `package.json` / `build.gradle`, commits, tags, creates GitLab or GitHub release
 
 ## Version bump rules
 
@@ -115,6 +115,12 @@ node:                                                # opt-in — no default; om
   #   - "packages/frontend/package.json"
   #   - "packages/backend/package.json"
 
+gradle:                                              # opt-in — no default; omit to skip
+  # build_file: "build.gradle"                       # Groovy or Kotlin DSL; single path
+  # build_files:                                     # multi-module: list overrides build_file
+  #   - "build.gradle"
+  #   - "module-a/build.gradle"
+
 gitlab:
   url: "https://gitlab.example.com"                  # or env CI_SERVER_URL
   token: ""                                          # env GITLAB_TOKEN (never commit this)