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.

dispatch/filters/chatmember/chatmember_test.go

@@ -12,15 +12,15 @@ func memberUpdate(status string, fromID int64) *api.ChatMemberUpdated {
 	var newMember api.ChatMember
 	switch status {
 	case "member":
-		newMember = &api.ChatMemberMember{Status: api.ChatMemberMemberStatusMember}
+		newMember = &api.ChatMemberMember{Status: api.ChatMemberStatusMember}
 	case "administrator":
-		newMember = &api.ChatMemberAdministrator{Status: api.ChatMemberAdministratorStatusAdministrator}
+		newMember = &api.ChatMemberAdministrator{Status: api.ChatMemberStatusAdministrator}
 	case "kicked":
-		newMember = &api.ChatMemberBanned{Status: api.ChatMemberBannedStatusKicked}
+		newMember = &api.ChatMemberBanned{Status: api.ChatMemberStatusKicked}
 	case "left":
-		newMember = &api.ChatMemberLeft{Status: api.ChatMemberLeftStatusLeft}
+		newMember = &api.ChatMemberLeft{Status: api.ChatMemberStatusLeft}
 	default:
-		newMember = &api.ChatMemberMember{Status: api.ChatMemberMemberStatusMember}
+		newMember = &api.ChatMemberMember{Status: api.ChatMemberStatusMember}
 	}
 	return &api.ChatMemberUpdated{
 		From:          api.User{ID: fromID},
@@ -70,7 +70,7 @@ func TestComposedFilters(t *testing.T) {
 func TestNewStatus_Owner(t *testing.T) {
 	u := &api.ChatMemberUpdated{
 		From:          api.User{ID: 1},
-		NewChatMember: &api.ChatMemberOwner{Status: api.ChatMemberOwnerStatusCreator},
+		NewChatMember: &api.ChatMemberOwner{Status: api.ChatMemberStatusCreator},
 	}
 	require.True(t, cmfilter.NewStatus("creator")(u))
 	require.False(t, cmfilter.NewStatus("member")(u))
@@ -79,7 +79,7 @@ func TestNewStatus_Owner(t *testing.T) {
 func TestNewStatus_Restricted(t *testing.T) {
 	u := &api.ChatMemberUpdated{
 		From:          api.User{ID: 1},
-		NewChatMember: &api.ChatMemberRestricted{Status: api.ChatMemberRestrictedStatusRestricted},
+		NewChatMember: &api.ChatMemberRestricted{Status: api.ChatMemberStatusRestricted},
 	}
 	require.True(t, cmfilter.NewStatus("restricted")(u))
 	require.False(t, cmfilter.NewStatus("member")(u))