fast-xdebug 0.3.0 — saturation fast-path

Performance: a saturation fast-path short-circuits the per-opcode handler once an op_array is fully covered. Line coverage on the benchmark is ~1.9x faster than 0.2.0 (0.031s vs 0.058s) — now below pcov and ~4.8x faster than Xdebug — while still doing branch/path that pcov can't. Adds opt-in ./configure --enable-fast-xdebug-native (-O3 -march=native; default build stays portable). Line output byte-identical to pcov/Xdebug; valgrind-clean; 9/9 tests on PHP 8.2-8.5.