put_cost reads the P95 from a fixed key while copying the percentile dynamically #111

src/probe.c:1573 copies percentile from the prober but reads the value from nsPerSampleP95. If clap-qualify's COST_PERCENTILE ever moved, every point's cost would vanish silently. Read the key from the percentile value, or refuse when percentile != 95.

Found in the review of #108.