Multiple percentile retrievals in single pass

[mwsis]

• batch percentile queries — p99_histogram_values_at_percentiles and p99_histogram_values_at_fixed_percentiles, with result types p99_pr_fp_result_t and p99_pr_fixed_results_t (80-byte fixed set);
• single-pass bucket walks for both batch APIs (fixed integer ranks and floating-point levels resolved in one cumulative scan);
• sequential semantics for p99_histogram_values_at_percentiles — levels should be ascending; a later target rank not greater than an earlier one receives the earlier result;
• benchmarks for batch percentile retrieval; unit tests including non-monotonic level order;
• ABI.md and README.md updated for new types and symbols;
• public header reorganised (types before functions; nested section banners);