Add opt-in streaming memory probes

[mikecerisano]

These Mac engine tests exercise two streaming paths with disposable 512 MiB sparse files and sample the process's physical memory footprint. The Standard copy probe uses the real fan-out writer with one and two destinations, checks bytes written and output sizes, and fails if footprint grows by 128 MiB or more. A separate probe checks the byte-comparison path used by Paranoid folder Compare. Missing footprint samples fail the tests instead of silently passing.

Both tests skip in normal runs. Run each alone with BITMATCH_COPY_MEMORY_TEST=1 or BITMATCH_COMPARE_MEMORY_TEST=1 and the matching StreamingMemoryProbeTests test filter.

On the current branch, copy measured 6→10 MiB with one destination and 10→10 MiB with two; byte comparison measured 5→14 MiB over 128 progress callbacks. Both isolated probes passed. These are bounded-memory checks for these paths, not a reproduction or explanation of issue #23. Sparse APFS input, an engine test process, and direct calls to the writer/comparer do not cover the reporter's media, full app workflow, destination verification, MHL, or reports.

Local validation: both opt-in probes passed again after rebasing onto main feb06db5; the full engine gates passed on the immediately preceding main 3f43fc85 (XCTest: 148 tests, 19 skipped, zero failures; Swift Testing: 181 tests in 34 suites). The Mac app test suite passed on feb06db5 (** TEST SUCCEEDED **). The normal engine suites skip these opt-in probes; the separate focused runs above executed them.