Fixes the last four medium-severity findings from the Java SDK audit list.

• Zero defer overrides task delay: EnqueueDecision.defer(Duration.ZERO) (e.g. deferUntil of a past instant) documents "enqueue now", but withDelay short-circuited on non-positive delays and silently kept the task's baked-in delayMs. The delay is now always applied, clamped to non-negative. Regression test asserts a 60s-delay task deferred by zero schedules immediately.
• Gzip codec javadoc: the class doc told readers to place GzipCodec after a signing codec — since codecs decode in reverse, that order decompresses attacker-controlled bytes before verifying integrity. Corrected to match the (already-correct) test and annotation-driven ordering.
• Primitive handler payloads rejected at compile time: a void handle(int payload) task handler generated new TypeReference<int>() {} — invalid Java surfacing as a cryptic javac error in the generated companion. The processor now reports a clear diagnostic on the offending method.
• In-memory backend FIFO: claimNext picked an arbitrary job among equal priorities (hash-map iteration order), diverging from production's priority DESC, scheduled_at ASC. Jobs now carry a monotonic sequence and ties break FIFO. Regression test runs five same-priority jobs on a single-thread worker and asserts enqueue order.

./gradlew build green across all subprojects.

Summary by CodeRabbit

• Bug Fixes

  • Prevents invalid Java generation by rejecting task handlers that use primitive payload types.
  • Ensures zero delay means “enqueue now,” even when a task has its own built-in delay.
  • Improves job ordering so same-priority work runs predictably in enqueue order.

• Documentation

  • Clarified delay behavior and codec ordering guidance.