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>

docs/content/usage.md

@@ -39,6 +39,7 @@ releaser --verbose --dry-run
 | `--branch-pattern <regex>` | `^(?:.*/)?release/(\d+)\.(\d+)$` | Override branch pattern (two capture groups: major, minor) |
 | `--tag-prefix <prefix>` | `""` | Prefix for version tags (e.g. `v` → `v1.2.3`) |
 | `--pom <path>` | `pom.xml` | Path to pom.xml relative to repo root |
+| `--gradle <path>` | — | Override `gradle.build_file` from config |
 | `--changelog-file <path>` | `CHANGELOG.md` | Path to changelog file |
 | `--release-env-file <path>` | `release.env` | Path for dotenv artifact; pass `""` to disable |
 | `--no-commit` | false | Update version files but stop before committing |