fix(cli): Defer zero-duration chunk adjustment

[leongdl]

What was the problem/requirement? (What/Why)

Adaptive chunk sizing divides the cumulative task duration by the number of completed items. A task that completes below the clock's measurable resolution can produce a cumulative duration of 0.0. With a positive target runtime, the next calculation produces infinity, and converting that value to usize saturates to usize::MAX. The iterator then attempts to place the remaining parameter range into one enormous chunk.

Call stack

openjd run
  -> run::execute
  -> execution::execute
  -> run_workload
  -> execute_step
  -> execute_step_tasks
  -> execute_parameter_space_tasks
       -> RunContext::run_task
          -> Instant::elapsed().as_secs_f64()
       -> adjust_adaptive_chunk_size
          -> target_runtime_seconds / duration_per_task
          -> StepParameterSpaceIterator::set_chunks_default_task_count

What was the solution? (How)

Extract the numeric calculation into calculate_adaptive_chunk_size and defer adjustment when the cumulative duration is non-positive or non-finite. The iterator retains its current chunk size until a measurable timing sample is available, avoiding an arbitrary epsilon and preventing usize::MAX from being installed.

The existing conservative blend for the first ten completed items is unchanged for measurable durations. Focused unit tests cover both zero-duration deferral and normal early-sample blending.

What is the impact of this change?

Sub-clock-resolution tasks no longer cause adaptive execution to collapse the remaining range into a single giant chunk. Normal adaptive chunk sizing is unchanged. There is no public API change.

How was this change tested?

• cargo test -p openjd-cli run::execution::tests — 2 passed
• cargo test -p openjd-cli --test cli_tests — 152 passed
• cargo test --workspace — passed
• cargo clippy --all-features --all-targets --workspace -- -D warnings — passed
• cargo fmt --all -- --check — passed
• OpenJD 2023-09 conformance suite — 1,162 passed, 0 failed

Was this change documented?

Yes. specs/cli/run.md now documents that adaptive sizing waits for a measurable duration before changing the chunk size.

Is this a breaking change?

No.

Does this change impact security?

No.

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