docs: AGENTS.md claims inline FFI::cdef declarations but the code loads ffi/zvec_ffi_php.h at runtime

[s2x]

Finding (from #192 follow-up)

AGENTS.md (FFI-Specific Rules) states: Inline C declarations in FFI::cdef() — do not load the .h file at runtime. — but the code does exactly the opposite.

Evidence

• src/ZVec.php:76-87 — FFI::cdef(trim($headerContent), $libPath) where $headerContent comes from file_get_contents of ffi/zvec_ffi_php.h (pre-existing on main, verified).
• ffi/zvec_ffi_php.h is the runtime-loaded header — any FFI signature change must update it in sync with ffi/zvec_ffi.h (see related sync-check gap).

Impact

Contributors following AGENTS.md may edit only ffi/zvec_ffi.h and miss the runtime-loaded header, producing a runtime TypeError (or ABI crash) instead of a build failure.

Suggested fix

Update the AGENTS.md FFI-Specific Rules section to document the actual mechanism (runtime cdef from ffi/zvec_ffi_php.h) and call out that both ffi/zvec_ffi.h and ffi/zvec_ffi_php.h must be kept in sync with ffi/zvec_ffi.cc.

Found during #192 implementation; verified by subagent review (CONFIRMED; no duplicate among all 105 issues).