feat(lsp): offer and document the registered extended mnemonics

Completion merges the extension registry's mnemonics beside the toolchain
entries, with operand shapes measured against the layer's own encoder, and
hover documents a registered mnemonic from its metadata: the extension
notice, the forms, the features, the fixed encodings and the manual
references.

Assisted-by: GLM 5.3 Flash

lsp/handlers.go

@@ -114,6 +114,9 @@ func (s *Server) completion(p completionParams) []CompletionItem {
 		}
 		items = append(items, CompletionItem{Label: r.Name, Kind: kind, Detail: r.Desc})
 	}
+	// The extended-instruction layer, beside the toolchain entries: the
+	// registered mnemonics with the operand shapes the registry accepts.
+	items = applyExtensionCompletion(items, a.Arch)
 	// Local labels defined in the document.
 	if f, _ := parser.Parse("", s.docs[p.TextDocument.URI]); f != nil {
 		for _, name := range labelNames(f) {
@@ -134,6 +137,7 @@ func (s *Server) hover(p hoverParams) *Hover {
 	a := arch.ForArch(arch.FromFilename(uriPath(p.TextDocument.URI)))
 
 	var md string
+	ext := extensionHover(a.Arch, word)
 	// Directives first: several of their names (TEXT, PCALIGN, FUNCDATA,
 	// the BYTE family) also sit in the generated instruction tables with
 	// empty summaries, and the directive documentation is the answer a
@@ -142,6 +146,13 @@ func (s *Server) hover(p hoverParams) *Hover {
 		md = doc
 	} else if in, ok := a.Lookup(word); ok {
 		md = "**" + in.Name + "**: " + in.Summary
+		// A mnemonic the extension layer registers documents its registered
+		// forms beside the table entry.
+		if ext != "" {
+			md += "\n\n" + ext
+		}
+	} else if ext != "" {
+		md = ext
 	} else if r, ok := a.Register(word); ok {
 		md = "**" + r.Name + "**: " + r.Class.String() + " register. " + r.Desc
 	} else if desc, ok := arch.PseudoRegDesc(word); ok {