refactor(repl): register 'categories' as HelpTopic with custom Render

Add a Render func() string field to HelpTopic that overrides the
default formatTopic rendering. Use it to register 'categories' as a
normal HelpTopic entry instead of a hardcoded if-branch in GetHelp.

- Remove hardcoded if topic == "categories" from GetHelp
- Remove hardcoded 'categories' from GetCompletionTopics
- Remove dead getCategoriesHelp function from help.go
- 'categories' now discovered through normal helpByTopic lookup

2 files changed, 40 insertions, 28 deletions

diff --git a/internal/repl/help.go b/internal/repl/help.go
index 02688e1..4c0691b 100644
--- a/internal/repl/help.go
+++ b/internal/repl/help.go
@@ -11,14 +11,10 @@ import (
 
 // GetHelp returns the formatted help text for a topic.
 // If topic is empty, it returns the general help overview.
-// If topic is "categories", it lists all categories.
 func GetHelp(topic string) string {
 	if topic == "" {
 		return getGeneralHelp()
 	}
-	if topic == "categories" {
-		return getCategoriesHelp()
-	}
 
 	// Look up by operator name or alias
 	topic = strings.ToLower(topic)
@@ -29,6 +25,9 @@ func GetHelp(topic string) string {
 	if !ok {
 		return fmt.Sprintf("No help for %q.\n\nType 'help' for an overview, or 'help categories' to list all topics.", topic)
 	}
+	if t.Render != nil {
+		return t.Render()
+	}
 	return formatTopic(t)
 }
 
@@ -43,14 +42,11 @@ func GetAllTopics() []string {
 }
 
 // GetCompletionTopics returns all help topics suitable for tab completion.
-// Includes operator names, aliases, and special topics like "categories".
+// Includes operator names and aliases from all registered HelpTopic entries.
 func GetCompletionTopics() []string {
 	seen := make(map[string]bool)
 	var topics []string
 
-	// Add "categories" as a special topic
-	topics = append(topics, "categories")
-
 	// Add all operator names (skip "help" itself — no need to complete help help)
 	for op, t := range helpByTopic {
 		if op == "help" {
@@ -131,26 +127,6 @@ func getGeneralHelp() string {
 	return sb.String()
 }
 
-// getCategoriesHelp lists all categories and their topics.
-func getCategoriesHelp() string {
-	var sb strings.Builder
-	sb.WriteString("Available help topics by category:\n\n")
-	for _, cat := range categoryOrder {
-		sb.WriteString(fmt.Sprintf("  %s:\n", cat))
-		for _, op := range helpByCat[cat] {
-			t := helpByTopic[op]
-			aliasStr := ""
-			if len(t.Aliases) > 0 {
-				aliasStr = " (" + strings.Join(t.Aliases, ", ") + ")"
-			}
-			sb.WriteString(fmt.Sprintf("    %-16s %s%s\n", op, t.Description, aliasStr))
-		}
-		sb.WriteString("\n")
-	}
-	sb.WriteString("Use 'help <topic>' for details on any topic.\n")
-	return sb.String()
-}
-
 // formatTopic returns the detailed help for a single HelpTopic.
 func formatTopic(t *HelpTopic) string {
 	var sb strings.Builder
diff --git a/internal/repl/help_topics.go b/internal/repl/help_topics.go
index 102f2c1..34bb708 100644
--- a/internal/repl/help_topics.go
+++ b/internal/repl/help_topics.go
@@ -3,6 +3,11 @@
 
 package repl
 
+import (
+	"fmt"
+	"strings"
+)
+
 type HelpTopic struct {
 	Category    string // e.g. "Arithmetic", "Stack", "Comparison", "Variables", "Hyper", "REPL"
 	Operator    string // the operator or command name (e.g. "+", "help", "rat")
@@ -10,6 +15,9 @@ type HelpTopic struct {
 	Description string
 	Usage       string // short usage hint
 	Examples    []string
+	// Render, when non-nil, overrides the default formatTopic rendering.
+	// Used for special topics like "categories" that need custom output.
+	Render func() string
 }
 
 // helpTopics is the single source of truth for all inline help entries.
@@ -456,6 +464,34 @@ var helpTopics = []HelpTopic{
 			"1 2.71828 [ln]  ln(1) + ln(2.71828) = 0 + 1 = 1",
 		},
 	},
+
+	// ── Special topics (custom render) ──
+
+	{
+		Category:    "REPL",
+		Operator:    "categories",
+		Description: "List all available help topics by category",
+		Usage:       "help categories",
+		Examples:    []string{"help categories    List all topics grouped by category"},
+		Render: func() string {
+			var sb strings.Builder
+			sb.WriteString("Available help topics by category:\n\n")
+			for _, cat := range categoryOrder {
+				sb.WriteString(fmt.Sprintf("  %s:\n", cat))
+				for _, op := range helpByCat[cat] {
+					t := helpByTopic[op]
+					aliasStr := ""
+					if len(t.Aliases) > 0 {
+						aliasStr = " (" + strings.Join(t.Aliases, ", ") + ")"
+					}
+					sb.WriteString(fmt.Sprintf("    %-16s %s%s\n", op, t.Description, aliasStr))
+				}
+				sb.WriteString("\n")
+			}
+			sb.WriteString("Use 'help <topic>' for details on any topic.\n")
+			return sb.String()
+		},
+	},
 }
 
 // buildHelpIndex builds lookup maps from helpTopics at package init.