fix(asm): carry the riscv64 U-type immediate raw

The toolchain writes the source immediate straight into imm[31:12]
(riscv64.s: AUIPC 24287, X10 encodes 7ffff517), and rejects values
beyond the signed 20-bit span; the encoder divided by 4096 instead and
truncated silently, so the high bits of every large AUIPC and LUI were
lost.

Assisted-by: GLM 5.3 Flash

asm/riscv_encode_test.go

@@ -109,6 +109,34 @@ TEXT ·imm(SB), NOSPLIT, $0
 	}
 }
 
+// TestRISCV_utypeImmediate pins the U-type immediate semantics byte for byte
+// against go tool asm (riscv64.s lines 76 to 85): the source immediate is the
+// raw 20-bit field, not a byte address to divide, and both sign extremes
+// encode.  The range beyond the signed 20 bits is the toolchain's own error.
+func TestRISCV_utypeImmediate(t *testing.T) {
+	fn := firstTextRISCV(t, `#include "textflag.h"
+TEXT ·utype(SB), NOSPLIT, $0
+	AUIPC $524287, X10
+	LUI $524287, X15
+	LUI $167, X15
+	AUIPC $-524288, X15
+	RET
+`)
+	code := assembleRISCVHelper(t, fn)
+	want := "17f5ff7f" + "b7f7ff7f" + "b7770a00" + "97070080"
+	if got := hex.EncodeToString(code[:16]); got != want {
+		t.Errorf("U-type immediates: %s, want %s", got, want)
+	}
+	fn = firstTextRISCV(t, `#include "textflag.h"
+TEXT ·wide(SB), NOSPLIT, $0
+	AUIPC $524288, X10
+	RET
+`)
+	if _, _, _, _, _, _, err := assembleRISCV(fn); err == nil {
+		t.Error("AUIPC $524288: expected the 20-bit range error, got none")
+	}
+}
+
 func TestRISCV_branches(t *testing.T) {
 	fn := firstTextRISCV(t, `#include "textflag.h"
 TEXT ·br(SB), NOSPLIT, $0