Add StreamingScannerPerformanceTests (issue #4), gated behind
PUREYAML_STREAMING_PERF_MEASURE=1, measuring scanStep() against scanAll() on
the largest real-world fixture (9.5MB) across a spread of chunk sizes.
Small chunk sizes (16-256 scalars) show a real, chunk-size-dependent rescan
cost, up to 6x at 16, consistent with the checkpoint-and-retry design
rescanning a fetch attempt once per chunk it spans. A control run with
isFinal: true from construction, where ranDry can never be set and no
rollback ever happens, still cost ~1.5x over scanAll(), proving a separate,
chunk-size-independent cost dominates at any realistic chunk size (4KB+): the
checkpoint was being copied on every fetch even when it could never be used.
Fixed by skipping the checkpoint entirely when reader.isFinal is true.
Re-measured rather than assumed fixed: the isolated control improved from
1.49x to 1.38x, a real but modest gain, while the large-chunk-size ratios
were unchanged within measurement noise. The remaining floor's cause is not
identified and is left open on issue #4 rather than patched further on
speculation; closing it needs a profiler, not another guess.
Whole-suite conformance (402/402) and the chunk-size differential proof both
still pass unchanged.
Mihaela Mihaljevic
7add6f0087