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

Assisted-by: GLM 5.3 Flash

lint/lint.go

@@ -96,6 +96,7 @@ const (
 	CodeDataStringWidth       = "data-string-width"
 	CodeDataNoGlobl           = "data-without-globl"
 	CodeDataExceedsGlobl      = "data-exceeds-globl"
+	CodeExtensionForm         = "extension-form"
 )
 
 // knownTextFlags are the flags recognised by runtime/textflag.h, plus the
@@ -327,13 +328,14 @@ func lintText(t *ast.Text, tab *arch.Table, archKnown bool, cfg Config, macros m
 				hasRet = true
 			}
 
+			// The extended-instruction layer is a mnemonic's second home:
+			// one registered there (the arm64 SVE family and the amd64
+			// BF16, VP2INTERSECT, FP16 and VNNI-INT16 families) is known,
+			// and it encodes through the extension registry rather than
+			// the encoder mirror the unencodable rule consults, so the
+			// wired extensions are never flagged as a gap of the table.
+			_, extended := asm.LookupExtension(cfg.Arch, mnem)
 			if archKnown && !cfg.Disable[CodeUnknownInstr] && !pseudoOps[upper] && !isMacroInvocation(mnem, macros) {
-				// The extended-instruction layer is a mnemonic's second home:
-				// one registered there (arm64's SVE family today) is known,
-				// and it encodes through the extension registry rather than
-				// the encoder mirror the unencodable rule consults, so the
-				// wired extensions are never flagged as a gap of the table.
-				_, extended := asm.LookupExtension(cfg.Arch, mnem)
 				if _, ok := tab.Lookup(mnem); !ok {
 					if !extended {
 						out = append(out, Diagnostic{
@@ -359,6 +361,15 @@ func lintText(t *ast.Text, tab *arch.Table, archKnown bool, cfg Config, macros m
 				}
 			}
 
+			// The amd64 extension layer carries the sharper diagnostic: a
+			// statement pinned to the registry is validated by encoding it,
+			// and the layer's own refusal (a mask on a scalar form, rounding
+			// on a VL form, k0 as a write mask) surfaces as the finding, the
+			// same text the assembler prints at build time.
+			if cfg.Arch == arch.AMD64 && amd64ExtPinned(upper) && !cfg.Disable[CodeExtensionForm] {
+				out = append(out, checkAmd64Extension(st, upper)...)
+			}
+
 			if archKnown && !cfg.Disable[CodeOperandCount] && !isMacroInvocation(mnem, macros) && !maskedEvex(mnem, st.Operands) {
 				if in, ok := tab.Lookup(mnem); ok && in.MinOps >= 0 {
 					n := len(st.Operands)