feat(doctor): check for Java (javac) as an optional tool - #668

What

Adds Java (javac) to concore doctor as an optional tool.

Why

mkconcore.py supports Java nodes (JAVACEXE/JAVACWIN/JAVAEXE/JAVAWIN, concore.java,
Dockerfile.java), but doctor did not check for a JDK, so a missing JDK only showed up
as a failure during build.

Details

• A missing JDK is reported as a warning (same as MATLAB/Verilog/Docker), not an error.
• Doctor also confirms javac actually runs. On macOS without a JDK, /usr/bin/javac is a
  stub that exists on PATH but exits non-zero, so a PATH check alone reports Java as present.
• The optional-tool list is now an OPTIONAL_TOOLS constant instead of an inline tuple.
• Added tests for: tool definition, config keys matching mkconcore.py, missing javac,
  failing stub javac, and working javac.

Testing

• pytest tests/test_doctor.py tests/test_setup.py passes.
• ruff format and ruff check are clean.
• Manually checked concore doctor on macOS (Apple Silicon, Python 3.14): Java shows as a warning.
• Note: tests/test_shm_abort.py and tests/test_shm_lifecycle.py (6 tests) fail on macOS both
  before and after this change. They are unrelated and I have left them alone.