enhancement: guard against drift between ffi/zvec_ffi.h and ffi/zvec_ffi_php.h (duplicated declarations)

[s2x]

Finding (from #192 follow-up)

ffi/zvec_ffi.h and ffi/zvec_ffi_php.h contain duplicated function declarations. The latter is the one loaded at runtime by FFI::cdef() (src/ZVec.php:76-87); the former documents/holds the same ABI. There is no check that they stay in sync.

Evidence

• ffi/zvec_ffi.h:331-334 vs ffi/zvec_ffi_php.h:253-256 — identical zvec_collection_fetch declarations, maintained by hand (both had to be updated for fetch(): expose includeVector + mirror upstream nullable normalization #192).
• Drift currently surfaces only as a runtime TypeError (arg count mismatch) or an ABI-level crash when a stale/short declaration shifts argument registers (e.g. an out-param read from the wrong register).

Suggested scope

• A build-time or CI check that extracts function signatures from both headers and diffs them (script or unit test), or
• generate ffi/zvec_ffi_php.h from ffi/zvec_ffi.h (single source of truth), or
• at minimum: a comment in both headers warning they must be changed together.

Possibly related to closed #83 (SMELL-002) — checked, not a duplicate.

Found during #192 code review; verified by subagent review (CONFIRMED).