Fix ABI fragility: compile bindings against the real llama.h instead of hand-copied ctypes structs

llama_capi.py previously hand-copied struct field order/types from one
specific llama.cpp commit's llama.h into Python ctypes Structures. A
different llama.cpp build could silently reorder or resize those fields and
corrupt memory rather than raising any error.

Replaced with a cffi "API mode" extension (build_capi.py) that #includes the
user's actual llama.h and links against their actual libllama.so. Struct
layout now comes from real compilation - a genuinely incompatible field
fails the build loudly instead of corrupting memory at runtime. Verified
byte-for-byte identical generation output against the prior ctypes
implementation at a fixed seed, plus the fork/peek logit round-trip check
(0.000000 max diff).

Requires a one-time `python build_capi.py` setup step (needs a C compiler,
which building llama.cpp itself already requires).

requirements.txt

@@ -3,3 +3,5 @@ gguf>=0.16
 jinja2>=3.1
 fastapi>=0.110
 uvicorn>=0.30
+cffi>=1.16
+setuptools>=68