Tests: Add the integer-signedness C reference

Eight cases, each reading operands from .bss globals (so clang cannot fold)
and writing its result to a .bss global (so the runtime proof is a bpftool
map dump). The IR clang emits is the specification for the signedness work:

    s64 = u32           zext i32 -> i64        (widening is source-driven)
    u64 = s32           sext i32 -> i64
    u32 / s32           udiv i32               (usual arithmetic conversions)
    u64 > s64           icmp ugt i64
    u32 * u32 -> u64    mul i32, then zext     (wraps at 32 before widening)
    u32 >> 4            lshr i32
    s32 >> 4            ashr i32

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BSDVsZH5NtoASyxB8FCtGU

Author: r41k0u

tests/c-form/signedness.bpf.c

@@ -0,0 +1,47 @@
+/* Reference for integer signedness. Each case reads its operands from .bss
+ * globals (so clang cannot constant-fold) and writes the result to a .bss
+ * global (so the runtime proof is a `bpftool map dump`). The IR clang emits
+ * for this file is the specification: zext vs sext on widening, udiv vs sdiv,
+ * icmp ugt vs sgt, lshr vs ashr, and the trunc/zext pair that makes
+ * u32 * u32 wrap at 32 bits. */
+#define SEC(name) __attribute__((section(name), used))
+typedef unsigned int __u32;
+typedef int __s32;
+typedef unsigned long long __u64;
+typedef long long __s64;
+
+/* inputs */
+__u32 u32_max = 0xFFFFFFFFu;
+__s32 s32_neg = -1;
+__u32 u32_ten = 10;
+__s32 s32_neg2 = -2;
+__u64 u64_ten = 10;
+__s64 s64_neg = -1;
+__u32 u32_half = 0x80000000u;
+__u32 u32_two = 2;
+
+/* results */
+__s64 widen_unsigned;   /* s64 = u32  -> 4294967295          zext      */
+__u64 widen_signed;     /* u64 = s32  -> 0xffffffffffffffff  sext      */
+__s32 reinterpret;      /* s32 = u32  -> -1                  no-op     */
+__u32 mixed_div;        /* u32 / s32  -> 0                   udiv i32  */
+__u64 unsigned_cmp;     /* u64 > s64  -> 0                   icmp ugt  */
+__u64 narrow_wrap;      /* u32 * u32  -> 0 (wraps at 32)     trunc     */
+__u32 shr_unsigned;     /* u32 >> 4   -> 0x0FFFFFFF          lshr      */
+__s32 shr_signed;       /* s32 >> 4   -> -1                  ashr      */
+
+SEC("tracepoint/raw_syscalls/sys_enter")
+int prog(void *ctx)
+{
+    widen_unsigned = u32_max;
+    widen_signed = s32_neg;
+    reinterpret = u32_max;
+    mixed_div = u32_ten / s32_neg2;
+    unsigned_cmp = u64_ten > s64_neg;
+    narrow_wrap = u32_half * u32_two;
+    shr_unsigned = u32_max >> 4;
+    shr_signed = s32_neg >> 4;
+    return 0;
+}
+
+char _license[] SEC("license") = "GPL";