[codex] feat(mapper): support Maven Spring projects

[julianshess]

Summary

• Add deterministic Maven detection, default Maven commands, and Maven mapper registration.
• Introduce Maven module/source/test features and reuse shared JVM/Spring role mapping for Gradle and Maven.
• Document Maven/Spring mapping output and make Windows test fixtures portable while keeping POSIX-only coverage active on POSIX platforms.

Impact

• Maven and Spring Boot projects can now be discovered through pom.xml/mvnw, including nested and multi-module Maven layouts.
• --project <artifactId-or-root> can target Maven modules through project:* and project-root:* tags.
• Windows contributors can run the test suite without failures from POSIX-only filesystem assumptions.

Test Plan

• oxfmt --check README.md docs/feature-mapping.md docs/index.md docs/quickstart.md src/detect.ts src/mapper.ts src/mapper.test.ts src/mappers/gradle.ts src/mappers/jvm.ts src/mappers/maven.ts src/workflow.test.ts
• corepack pnpm typecheck
• corepack pnpm lint
• corepack pnpm test (8 files, 530 passed, 17 skipped on Windows)
• corepack pnpm build
• git diff --check