refactor(api): unify per-variant single-value enums into shared union-level enums

Each sealed-interface union with N variants used to emit N typed-string
enums, one per variant, each holding exactly one wire value. Codegen now
detects this pattern and emits ONE unified enum at the union level,
retyping every variant's discriminator field to point at it.

Unified enums (11 unions, 44 constants total):
  - ChatMemberStatus           (6)
  - MessageOriginType          (4)
  - BackgroundFillType         (3)
  - BackgroundTypeKind         (4)
  - ChatBoostSourceKind        (3)
  - OwnedGiftType              (2)
  - PaidMediaType              (4)
  - ReactionTypeKind           (3)
  - RevenueWithdrawalStateKind (3)
  - StoryAreaTypeKind          (5)
  - TransactionPartnerType     (7)

Naming-collision cases (union name already ends in a discriminator
concept noun, so the natural concat would stutter): BackgroundType,
ReactionType, StoryAreaType, ChatBoostSource, RevenueWithdrawalState.
The unified name ends with 'Kind' instead.

44 obsolete per-variant single-value enum types removed (e.g.
ChatMemberOwnerStatus, MessageOriginUserType). The variant struct types
themselves (ChatMemberOwner etc.) are unchanged; only their per-variant
single-value enum aliases go away.

Auto-inject MarshalJSON (commit 370c9c0) is unaffected — variant wire
values still come from the discriminator-extractor pass.

Call-site cleanup:
  - dispatch/filters/chatmember/chatmember_test.go
  - api/marshaljson_variants_test.go

New test: api/unifiedenum_test.go covers ChatMemberStatus and
MessageOriginType: variant-field retype, direct comparison without
conversion, marshal discriminator preservation, full round-trip,
stutter-suffix Kind sanity check.

cmd/genapi/enums.go

@@ -41,12 +41,25 @@ func planEnums(api *spec.API) *enumPlan {
 		valueKey  string // canonical key for value-set dedup
 	}
 
+	// Unification pass: for each sealed-interface union, fold per-variant
+	// single-value enum fields that share a discriminator name into ONE
+	// unified enum at union level. Claimed (parent,fieldName) tuples are
+	// excluded from the per-field grouping below.
+	unifiedDecls, unifiedByField := planUnifiedUnionEnums(api)
+	claimed := func(parent, fieldName string) bool {
+		_, ok := unifiedByField[enumKey(parent, fieldName)]
+		return ok
+	}
+
 	var refs []ref
 	collect := func(parent string, fields []spec.Field) {
 		for _, f := range fields {
 			if len(f.EnumValues) == 0 {
 				continue
 			}
+			if claimed(parent, f.Name) {
+				continue
+			}
 			refs = append(refs, ref{
 				parent:    parent,
 				fieldName: f.Name,
@@ -190,9 +203,172 @@ func planEnums(api *spec.API) *enumPlan {
 		plan.decls[g.name] = enumDecl{Name: g.name, Values: g.values}
 		_ = vk
 	}
+	// Merge unified union enums (already named with stutter handling and
+	// keyed per-variant in unifiedByField).
+	for k, name := range unifiedByField {
+		plan.byField[k] = name
+	}
+	for name, d := range unifiedDecls {
+		plan.decls[name] = d
+	}
 	return plan
 }
 
+// planUnifiedUnionEnums detects sealed-interface unions whose variants
+// share a single discriminator field with one enum value each, and emits
+// ONE unified enum per union covering all variant values. Returns the
+// declarations to emit and the per-(variant,fieldName) map to point each
+// variant's field at the unified enum.
+//
+// A union qualifies when EVERY variant in t.OneOf:
+//  1. defines a field with the same Go-name (e.g. "Status", "Type", "Source");
+//  2. that field is a required string with len(EnumValues)==1.
+//
+// The picked Go-name is the first one tried in this priority order:
+//   - knownDiscriminators[union].Field's Go-name (resolved via JSONName match);
+//   - "Type", "Status", "Source" (the three discriminators Telegram uses).
+//
+// First match wins; if none qualify, the union is skipped (variants keep
+// their existing per-field treatment, which still single-emits via the
+// regular grouping pass).
+func planUnifiedUnionEnums(api *spec.API) (map[string]enumDecl, map[string]string) {
+	decls := map[string]enumDecl{}
+	byField := map[string]string{}
+
+	typeByName := make(map[string]*spec.TypeDecl, len(api.Types))
+	for i := range api.Types {
+		typeByName[api.Types[i].Name] = &api.Types[i]
+	}
+
+	// Iterate unions in deterministic (declaration) order.
+	for ui := range api.Types {
+		u := &api.Types[ui]
+		if len(u.OneOf) == 0 {
+			continue
+		}
+
+		// Resolve the variants. Skip unions where any variant is missing
+		// (defensive — shouldn't happen in a well-formed IR).
+		variants := make([]*spec.TypeDecl, 0, len(u.OneOf))
+		for _, vName := range u.OneOf {
+			v, ok := typeByName[vName]
+			if !ok {
+				variants = nil
+				break
+			}
+			variants = append(variants, v)
+		}
+		if len(variants) == 0 {
+			continue
+		}
+
+		// Build the candidate Go-name list. Priority order:
+		//  1. discriminator GoField from knownDiscriminators (resolved via JSONName);
+		//  2. "Type", "Status", "Source".
+		var candidateNames []string
+		seen := map[string]bool{}
+		add := func(name string) {
+			if name == "" || seen[name] {
+				return
+			}
+			seen[name] = true
+			candidateNames = append(candidateNames, name)
+		}
+		if ds, ok := knownDiscriminators[u.Name]; ok && ds.Field != "" {
+			// Resolve Go-name from the first variant whose field matches the JSON name.
+			for _, v := range variants {
+				for _, f := range v.Fields {
+					if f.JSONName == ds.Field {
+						add(f.Name)
+						break
+					}
+				}
+			}
+		}
+		for _, n := range []string{"Type", "Status", "Source"} {
+			add(n)
+		}
+
+		// Find the first candidate Go-name where every variant has a
+		// matching single-value string-enum field.
+		var (
+			pickedName string
+			pickedDocs map[string]spec.Field // variant name -> field
+		)
+		for _, name := range candidateNames {
+			matches := map[string]spec.Field{}
+			ok := true
+			for _, v := range variants {
+				var hit *spec.Field
+				for fi := range v.Fields {
+					if v.Fields[fi].Name == name {
+						hit = &v.Fields[fi]
+						break
+					}
+				}
+				if hit == nil ||
+					hit.Type.Kind != spec.KindPrimitive ||
+					hit.Type.Name != "string" ||
+					len(hit.EnumValues) != 1 {
+					ok = false
+					break
+				}
+				matches[v.Name] = *hit
+			}
+			if ok {
+				pickedName = name
+				pickedDocs = matches
+				break
+			}
+		}
+		if pickedName == "" {
+			continue
+		}
+
+		// Build the unified enum name with stutter handling.
+		enumName := unifiedEnumName(u.Name, pickedName)
+
+		// Collect values across variants in deterministic order, deduping.
+		valueOrder := make([]string, 0, len(variants))
+		valueSeen := map[string]bool{}
+		for _, v := range u.OneOf {
+			f := pickedDocs[v]
+			val := f.EnumValues[0]
+			if valueSeen[val] {
+				continue
+			}
+			valueSeen[val] = true
+			valueOrder = append(valueOrder, val)
+		}
+
+		decls[enumName] = enumDecl{Name: enumName, Values: valueOrder}
+		for _, v := range variants {
+			byField[enumKey(v.Name, pickedName)] = enumName
+		}
+	}
+
+	return decls, byField
+}
+
+// unifiedEnumName builds the union-level enum name. Falls back to a
+// "Kind" suffix when the naive concatenation reads as a stutter:
+//
+//   - union name ends in the field name verbatim (e.g. BackgroundType+Type);
+//   - union name ends in any "concept noun" — Type/Status/Source/State —
+//     so appending another such noun would duplicate the suffix
+//     (e.g. ChatBoostSource+Source, RevenueWithdrawalState+Type).
+//
+// Otherwise the natural concatenation wins (ChatMember+Status →
+// ChatMemberStatus, MessageOrigin+Type → MessageOriginType).
+func unifiedEnumName(unionName, fieldName string) string {
+	for _, suf := range []string{"Type", "Status", "Source", "State"} {
+		if strings.HasSuffix(unionName, suf) {
+			return unionName + "Kind"
+		}
+	}
+	return unionName + fieldName
+}
+
 // All returns the enum declarations sorted by name for deterministic emit.
 func (p *enumPlan) All() []enumDecl {
 	out := make([]enumDecl, 0, len(p.decls))