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/handlers.go

@@ -1127,6 +1127,13 @@ func (s *Server) diagnosticsFor(uri string) []Diagnostic {
 	cfg := lint.Config{Arch: arch.FromFilename(uriPath(uri))}
 	diags := lint.File(f, cfg)
 
+	// The extension layer owns the verdict on its own mnemonics: the registry
+	// encodes them, so a report of unknown or unencodable against a
+	// registered mnemonic is the generated table's ignorance, not the file's
+	// defect.  The filter consults the registry directly and is a no-op once
+	// lint learns the registry itself, so the two compose unchanged.
+	owned := extensionMnemonicSites(f, cfg.Arch)
+
 	out := make([]Diagnostic, 0, len(diags)+len(errs))
 	for _, e := range errs {
 		pos := token.Position{Line: 1, Column: 1}
@@ -1142,6 +1149,9 @@ func (s *Server) diagnosticsFor(uri string) []Diagnostic {
 		})
 	}
 	for _, d := range diags {
+		if extOwnedCode(d.Code) && owned[[2]int{d.Pos.Line, d.Pos.Column}] {
+			continue
+		}
 		out = append(out, Diagnostic{
 			Range:    clientRange(text, toRange(d.Pos.Line, d.Pos.Column, d.End)),
 			Severity: lintSeverity(d.Severity),