[rustjava-runtime-time-todo-impl] fix: implement time APIs in RuntimeImpl + regression fixture

[Jun025]

What

The only production Runtime implementation (RuntimeImpl, the sole path used by the rust_java binary via src/lib.rs) had now(), sleep(), and r#yield() left as todo!(). Any Java program touching System.currentTimeMillis(), Thread.sleep(), Thread.yield(), new Date(), Timer, or Object.wait(long) died with a Rust panic instead of running. CI stayed green because the test_data corpus used none of these APIs and TestRuntime had its own implementations.

Changes

• src/runtime.rs: now() → epoch ms via SystemTime; sleep() → tokio::time::sleep; r#yield() → tokio::task::yield_now
• test_utils/src/lib.rs: fill the matching r#yield() todo!() so test/binary behavior stays identical
• Cargo.toml: add tokio time feature to both target sections (only change; no other features/versions touched)
• test_data/TimeApi.{class,txt}: new fixture (compiled --release 21) exercising currentTimeMillis / yield / sleep / Date through the RuntimeImpl integration path, with deterministic assertions only (positivity, monotonicity, ≥50ms elapsed after a 100ms sleep — no absolute-time or tight-timing checks)

Fixture source (repo convention keeps only .class + .txt):

import java.util.Date;

public class TimeApi {
    public static void main(String[] args) throws InterruptedException {
        long start = System.currentTimeMillis();
        System.out.println(start > 0 ? "currentTimeMillis positive" : "FAIL currentTimeMillis");
        Thread.yield();
        System.out.println("yield returned");
        Thread.sleep(100);
        long end = System.currentTimeMillis();
        System.out.println(end >= start ? "monotonic after sleep" : "FAIL monotonic");
        System.out.println(end - start >= 50 ? "sleep elapsed" : "FAIL sleep elapsed");
        Date date = new Date();
        System.out.println(date.getTime() > 0 ? "date positive" : "FAIL date");
    }
}

Evidence

Before fix (fixture added first — reproduces through RuntimeImpl):

thread 'test_class' panicked at src/runtime.rs:110:9:
not yet implemented

After fix:

• cargo test --all: all green, 0 failures (test_class now includes TimeApi: 1 passed)
• Deployed binary: cd test_data && rust_java TimeApi → all 5 assertion lines pass, exit 0
• cargo build, cargo fmt --check, cargo clippy --all-targets clean

Non-goals (per ticket)

• jvm_rust/src/interpreter.rs:629 remaining todo!()
• Timer/scheduling semantics beyond panic removal
• No upstream (dlunch/RustJava) contact

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