Upgrade to Java 17+, modernize build/CI, and refactor dynamic compiler internals

[raulgomis]

Improve Java compatibility, diagnostics, coverage, and documentation

Summary

This change strengthens the library's compatibility and quality checks, fixes
diagnostic state leaking between compilations, expands test coverage, and
provides clearer documentation for users and contributors.

Changes

• Target Java 17 bytecode so the library remains usable on Java 17 and later.
• Test the project in CI on Java 17, 21, 25, and 26.
• Run the Maven verify lifecycle in CI and upload the generated JaCoCo report
  for each tested JDK.
• Create a fresh diagnostic collector for every compilation so failures do not
  retain diagnostics from earlier attempts.
• Make diagnostic error output use a stable newline across operating systems.
• Add focused tests for:
  • independent diagnostics across compilation failures
  • named-package compilation
  • wrapped class-loading failures
  • dynamic class and resource loading
  • file-manager lookup, delegation, output, and binary-name handling
  • exception formatting, defensive copies, null diagnostics, and causes
• Expand the README with requirements, installation, usage, package and error
  examples, security guidance, and contributor instructions.
• Add repository-specific guidance for coding agents in AGENTS.md.

Verification

• mvn clean verify
• 26 tests pass
• 97.7% line coverage
• 91.7% branch coverage
• 97.7% instruction coverage
• git diff --check

Notes

Dynamically compiled code is not sandboxed and runs with the permissions of the
host application. The updated README now calls this out explicitly.