Skip tests at runtime

A test that needs an environment capability — hardware perf counters, a readable tracefs, a toolchain binary on PATH — should skip when the capability is missing, not silently pass: an early return counts a degraded environment as green and masks the gap. Call skipTest from sparkles.test_runner.skip (also re-exported by sparkles.test_runner):

import sparkles.test_runner : skipTest;

@("perf.counters.smoke")
@system unittest
{
    auto g = PerfGroup.tryOpen(true);
    if (!g.available)
        skipTest("hardware counters unavailable (perf_event_paranoid?)");
    // … the real test …
}

The runner records the test as skipped — neither passed nor failed:

 ⊘ pkg.perf perf.counters.smoke (hardware counters unavailable (perf_event_paranoid?))

Summary: 12 passed, 0 failed, 1 skipped in 4.1ms

• The N skipped summary segment appears only when something skipped, and a skip never fails the run — surfacing, not punishing, is the point.
• skipTest is @safe pure nothrow @nogc (it throws a recycled TestSkipped : Error), so the strictest test bodies can call it without relaxing their attributes.

Caveats

• Runtime only. A @ctfe body is evaluated by the compile-time probe — a skip there is a compile error, not a skip. The extracted --better-c/--wasm programs have no druntime classes; guard those bodies by other means.
• @benchmark bodies: prefer skipping at registration. skipTest at the top of the body marks the whole benchmark skipped (a ⊘ line, no rows). Inside a deferred benchIter/benchCase closure it skips only that case — a yellow row in its table — while the rest of the matrix measures and the run stays green.
• The mode-level toolchain skips (--better-c/--wasm/@ctfe with no compiler) remain stderr notes with exit 0; skipTest is for per-test environment probes.