[adamant-pwn]

With USE_JEMALLOC, Folly calls unprefixed allocator helpers even when jemalloc was built with a symbol prefix (as on conda-forge macOS). Weak declarations can let this compile while calls still resolve to missing functions. Bind these helpers through jemalloc's documented je_* aliases in Folly's portability header, leaving standard allocation names and automatic allocator detection unchanged.

Retain the je_* aliases with JEMALLOC_NO_DEMANGLE, including for consumers that include jemalloc first. Register the existing MallocTest suite with CMake and add an allocation/deallocation smoke test. This smoke test alone does not establish prefixed-allocator compatibility.

Tested: 101 selected allocator/vector tests pass with unprefixed jemalloc 5.4.0. A separate Linux link-and-run reproduction with prefixed jemalloc 5.4.0 crashes without the patch and passes with it. Mac header probes compile, but full native Mac and Rosetta runtime validation of this upstream version is pending.

Prepared with AI assistance; this PR remains a draft for review.

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