feat(lint): surface the amd64 extension layer's refusals

Assisted-by: GLM 5.3 Flash

lint/extension.go

@@ -0,0 +1,49 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The lint side of the extended-instruction layer.  The generated tables and
+// the extension registry are two homes for a mnemonic, so a registered
+// mnemonic is known wherever the tables are consulted, and a statement pinned
+// to the registry is validated by the registry's own encoding, whose refusal
+// is the sharper diagnostic the layer carries.
+
+package lint
+
+import (
+	"sourcedock.dev/petrbalvin/gasm-sdk/arch"
+	"sourcedock.dev/petrbalvin/gasm-sdk/asm"
+	"sourcedock.dev/petrbalvin/gasm-sdk/ast"
+)
+
+// amd64ExtPinned reports whether a statement belongs to the amd64 extension
+// layer alone: the mnemonic is registered in the registry and the encoder
+// mirror cannot encode it, the same routing the assembler applies before its
+// scalar paths run.  A statement the layer pins never passes through go tool
+// asm, so the per-operand portability advice the scalar statements earn is
+// noise on it.
+func amd64ExtPinned(upper string) bool {
+	if _, ok := asm.LookupExtension(arch.AMD64, upper); !ok {
+		return false
+	}
+	return !asm.Encodable(upper)
+}
+
+// checkAmd64Extension validates one amd64 statement pinned to the extension
+// layer by encoding it through the registry: the layer's own validators carry
+// the sharper diagnostic (a mask on a scalar form, rounding on a VL form, k0
+// as a write mask), and their refusal is the diagnostic, the same text the
+// assembler prints at build time.  A statement the registry encodes is clean:
+// the wired layer is the acceptance, not a gap of the generated table.
+func checkAmd64Extension(st *ast.Instr, upper string) []Diagnostic {
+	_, _, err := asm.EncodeAmd64Statement(upper, st.Operands)
+	if err == nil {
+		return nil
+	}
+	return []Diagnostic{{
+		Pos:      st.Mnemonic.Pos,
+		End:      st.Mnemonic.End,
+		Severity: Error,
+		Code:     CodeExtensionForm,
+		Message:  err.Error(),
+	}}
+}