The stack-backed one-shot decompression entry point used to make its own
heap-mode rejection, static DCtx initialization, allocation-error mapping,
and dispatch ordering in C. That kept scalar control flow beside the private
DCtx layout even though Rust already owns the decompression policy and context
leaf.
Rust now consumes an ABI-checked scalar projection and callback set. It makes
the branch and ordering decisions, maps a failed static initializer to the
same memory-allocation error, and invokes the existing C-owned context leaf.
The C shim retains the local stack object, private DCtx layout, and
`ZSTD_initStaticDCtx` implementation. Recording-callback tests cover heap
rejection, initializer failure, and successful initialization-to-dispatch
ordering without fabricating the private context layout.
Test Plan:
- `git diff --cached --check` -- passed
- `rustfmt --edition 2021 --check rust/src/zstd_decompress.rs` -- passed
- Full capped Rust/native verification remains pending until the parallel
compression seam is integrated.
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