feat(lsp): leave the extension verdicts to the registry

Diagnostics no longer repeat the generated table's ignorance of a
registered mnemonic: unknown-instruction and unencodable-instruction
against a statement the registry encodes are filtered from the server's
own presentation, driven by asm.LookupExtension directly.  The filter is
a no-op once lint learns the registry, so the two compose unchanged.

Assisted-by: GLM 5.3 Flash

lsp/extensions_test.go

@@ -283,3 +283,71 @@ func TestExtensionHoverWithoutProse(t *testing.T) {
 		}
 	}
 }
+
+// TestDiagnosticsExtensionOwnedARM64 pins the pass-through on arm64: the
+// lint on this base still reports a registered mnemonic as unknown, and the
+// server filters its own presentation through the registry, while a mnemonic
+// no layer registers stays reported.
+func TestDiagnosticsExtensionOwnedARM64(t *testing.T) {
+	diags := publishedDiagnostics(t, "file:///v_arm64.s", sveDoc)
+	for _, d := range diags {
+		if strings.Contains(d.Message, "SQADD") {
+			t.Errorf("registry mnemonic still reported: %+v", d)
+		}
+	}
+	unknown := 0
+	for _, d := range diags {
+		if d.Code == "unknown-instruction" {
+			unknown++
+			if !strings.Contains(d.Message, "NOSUCHTHING") {
+				t.Errorf("unknown-instruction against the wrong statement: %+v", d)
+			}
+		}
+	}
+	if unknown != 1 {
+		t.Errorf("unknown-instruction count = %d, want the one unregistered mnemonic", unknown)
+	}
+}
+
+// TestDiagnosticsExtensionOwnedFocused is the focused filter test: the
+// server's own diagnostic computation, without the protocol, answers nothing
+// against the registered mnemonics and keeps everything else.
+func TestDiagnosticsExtensionOwnedFocused(t *testing.T) {
+	srv := &Server{docs: map[string]string{"file:///v_arm64.s": sveDoc}}
+	diags := srv.diagnosticsFor("file:///v_arm64.s")
+	if len(diags) == 0 {
+		t.Fatal("diagnostics empty, want the unregistered mnemonic")
+	}
+	for _, d := range diags {
+		switch {
+		case strings.Contains(d.Message, "SQADD"):
+			t.Errorf("registry mnemonic still reported: %+v", d)
+		case d.Code == "unknown-instruction":
+			if !strings.Contains(d.Message, "NOSUCHTHING") {
+				t.Errorf("unexpected unknown-instruction: %+v", d)
+			}
+		}
+	}
+}
+
+// TestDiagnosticsExtensionAMD64StillReported is the per-architecture
+// negative case: on amd64 the registry owns nothing, so the same source
+// keeps every unknown-instruction report, the SVE mnemonics included.
+func TestDiagnosticsExtensionAMD64StillReported(t *testing.T) {
+	diags := publishedDiagnostics(t, "file:///v_amd64.s", sveDoc)
+	found := map[string]bool{}
+	for _, d := range diags {
+		if d.Code == "unknown-instruction" {
+			for _, mnem := range []string{"SQADD", "NOSUCHTHING"} {
+				if strings.Contains(d.Message, mnem) {
+					found[mnem] = true
+				}
+			}
+		}
+	}
+	for _, mnem := range []string{"SQADD", "NOSUCHTHING"} {
+		if !found[mnem] {
+			t.Errorf("amd64 lost the unknown-instruction report for %q: %+v", mnem, diags)
+		}
+	}
+}