test(runner): pin the clock probe to once per run, not once per worker

[Chemaclass]

🤔 Background

Related #1353

Resolving the clock implementation happened inside a $( ), so the resolved value died with that subshell and every --parallel worker re-probed. On a shell without EPOCHREALTIME the probe forks perl, so it scaled one-for-one with the tests — and fired even with per-test timing off, since deciding that timing is off is what asks whether the clock is expensive.

The behaviour is already fixed. #1358 converted clock::init to now_to_slot for its own reasons, which resolves the impl in the main shell before any worker is dispatched. Measured on a 500-test file, macOS arm64, bash 3.2.57:

init	--no-parallel	--parallel
old	3	502
new	2	2

💡 Changes

• Nothing pinned that, so this adds the two guards. The unit test is the invariant itself — init must leave _BASHUNIT_CLOCK_NOW_IMPL set in the caller. The acceptance test is the differential the issue asked for: eight times the tests must not cost more probes. Both fail against the old init (7 probes against 42).
• A differential rather than a budget, because a platform whose clock is EPOCHREALTIME or date forks no perl at all — comparing two sizes still fails loudly if the count starts tracking the test count.
• perf-fork-budget.md stated the parallel budget from a sequential census fixture, which cannot see a per-worker cost by construction. It now says so.

https://claude.ai/code/session_01EXYWTGLjf7qM8Ru3GakDRm