perf(runner): fold clock perl probe into the first read

[Chemaclass]

🤔 Background

Follow-up to the cold-start profiling in #798. Selecting the perl clock impl forked perl twice at cold start: an empty Time::HiRes verification probe, then a separate first time read.

💡 Changes

• Read the actual time during impl selection and seed the return slot, so the first now_to_slot reuses it instead of forking a second interpreter.
• A non-digit/empty result still falls through to the next impl, preserving the fallback chain.
• 2 → 1 perl fork per cold start on bash 3.2 / macOS (Linux/BSD use date, unaffected); regression test extended.