After posting, I realized an obvious difference: I'm now building the Apple Silicon version, not the Intel one, so the "existing binary" is running under Rosetta, but the compiled one is running apple silicon.
mackworth
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So, the ultimate answer proved to be totally different. After agonizing amounts of diagnosing, trying new configurations, I finally realized that there are two error messages up there:
Reason: no suitable image found. Did find:
file system relative paths not allowed in hardened programs
The first one turned out to be correct; there was, in fact, a dylib missing from my build. (One for x86_64 arch)
The second message is completely wrong, and led me down a long irrelevant path.