Adds the executors module: shutdown() vs shutdownNow(), the graceful
two-phase shutdown pattern, AutoCloseable/try-with-resources (JDK 19+),
newVirtualThreadPerTaskExecutor() timing vs a bounded pool, and the
submit()-without-get() lost-exception trap. 5 runnable demos, 1 JUnit
correctness test class (23 tests), 6 captured output transcripts.
Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM
