Keep the target-specific ZSTD_LEGACY_SUPPORT macro in the C translation unit, but pass its scalar value through a Rust policy bridge before legacy dispatch. Rust now accepts only the historical supported range 1 through 7 and normalizes all other values, including a missing projection, to disabled. ABI assertions and focused tests preserve the compile-time configuration contract.
Test Plan: Not run in this atomic commit; the capped serial Rust, native, smoke, and original test-suite verification follows.
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