bench(c_accel): drop defensive lua_type checks; refresh numbers

person_codec.c had a per-element lua_type(L,-1)==LUA_TSTRING check
inside the emails loop, plus field-level lua_type checks that
generic_codec.c skipped. At 100KB that's ~2700 extra C calls per
message in the hot path, which made S4 (hand-written) look slower
than S3 (generic) at 1KB+ and led to a wrong "branch prediction on
divergent paths" hypothesis in the original README.

Replaced the per-element check with no check and the field-level
lua_type checks with lua_isnil to match generic_codec.c's semantics.
S3 and S4 are now within +/-5% at every size, which strengthens the
ra6 architecture call (ship the generic one-call codec; codegen-
emitted per-message C buys nothing).

Also added bench/c_accel/compile_flags.txt so clangd resolves
<module.h> and friends -- mirrors runtime/pb/c/compile_flags.txt.