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>

ROADMAP.md

@@ -85,5 +85,5 @@
 
 ## Future / backlog
 
-- Gradle support (`build.gradle` / `build.gradle.kts`)
+- ~~Gradle support (`build.gradle` / `build.gradle.kts`)~~ — ✓ shipped v1.6.0 (`internal/gradle`; Groovy + Kotlin DSL; multi-module via `gradle.build_files`; `--gradle` flag)
 - Slack / Teams notification on release