ffi 0.1: primitives baseline (#import c, one roundtrip per type)

87/87 regression tests pass (was 86; +ffi-01-primitives).

vendors/ffi_primitives/{.h,.c} exposes a trivial identity roundtrip
per primitive C type — int/uint/short/ushort/long long/unsigned long
long/signed char/unsigned char/float/double/void* — plus two-arg
add helpers (int + double) for multi-arg ABI exercise. The sx-side
example imports the .h via `#import c { #include / #source }` and
prints each result; the snapshot in tests/expected pins today's
parameter + return ABI so Phase 1's #objc_call / #jni_call lowering
work can't silently regress primitive marshalling.

Two findings logged in current/CHECKPOINT-FFI.md's Known issues
section (current behavior, not new bugs): (1) c_import.zig maps
`signed char` -> `u8` not `s8`, and (2) sx integer-literal parser
rejects values >= 2^63 as overflow even when the receiver is u64.
Both worked around in this test without blocking the baseline.

tests/expected/ffi-01-primitives.exit

@@ -0,0 +1 @@
+0

tests/expected/ffi-01-primitives.txt

@@ -0,0 +1,13 @@
+ffi_id_int(-42)         = -42
+ffi_id_short(-1234)     = -1234
+ffi_id_s64(huge)        = 9000000000000000000
+ffi_id_uint(0xDEADBEEF) = 3735928559
+ffi_id_ushort(0xFFFF)   = 65535
+ffi_id_u64(0x7FEE...)   = 9218551421072305134
+ffi_id_schar(127)       = 127
+ffi_id_uchar(255)       = 255
+ffi_id_f32(3.5)         = 3.500000
+ffi_id_f64(1.5)         = 1.500000
+ffi_id_ptr roundtrip    = true
+ffi_add_int(7, 8)       = 15
+ffi_add_double(0.25, 0.75) = 1.000000