feat(api): add api.Ptr helper for optional scalar fields

Telegram's optional int/bool/float fields are pointers so callers can
explicitly send false or 0 to override a chat default — distinct from
'absent', which uses the chat default. The pointer construction has been
ergonomically painful:

  photoLimit := int64(5)
  Limit: &photoLimit

api.Ptr[T any](v T) *T collapses that to a single line:

  Limit: api.Ptr[int64](5)
  DisableNotification: api.Ptr(true)

Pointers stay because the explicit-zero distinction matters for fields
like DisableNotification, ProtectContent, and getUpdates.Offset where
sending 0 / false explicitly is semantically different from omitting
the field.

api/ptr.go

@@ -0,0 +1,16 @@
+package api
+
+// Ptr returns a pointer to v. Useful for optional scalar fields where
+// the wire format must distinguish absent (nil) from an explicit zero
+// value (e.g. DisableNotification: api.Ptr(false) to override the
+// chat default).
+//
+// For untyped literals, supply the type parameter explicitly:
+//
+//	Limit: api.Ptr[int64](5)
+//
+// For already-typed values, type inference handles it:
+//
+//	var n int64 = 5
+//	Limit: api.Ptr(n)
+func Ptr[T any](v T) *T { return &v }