96/96 regression tests pass (+ffi-09-foreign-result-chain).
Opaque C-handle pattern that mirrors how real sx code threads
MTLBuffer*, AAssetManager*, file pointers, etc. through composite
sx values. C side has a trivial heap-int handle (`ffi_chain_make`
returning `void*`, `ffi_chain_bump` / `_peek` / `_dispose`). The sx
side exercises:
1. Chained calls — make -> bump -> bump -> peek; one handle
threaded through four FFI sites in sequence.
2. Struct field — `Counter { handle: *void; label: string; }`
hosts the handle; methods/accesses go through
`.handle` to feed back into C.
3. List(*void) — push N handles, iterate, peek each, iterate
again to bump each, iterate again to read
back. Catches any aliasing / lifetime breakage
when handles round-trip through the slice
backing of List.
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