Unify column accessor APIs (column vs col DSL) before 1.0

[Jolanrensen]

Problem

There are multiple ways to create column accessors:

• constructors: column(), columnGroup()
• DSL: col(), colGroup(), ""<>, ""[]

These APIs overlap in functionality but differ in syntax and behavior.

As a result:

• API is inconsistent
• users have multiple ways to do the same thing
• discoverability and learning are harder

Design question

How should column accessor APIs be unified?

Options:

• keep one approach and deprecate the other
• keep both but make them behave identically
• standardize naming and usage across all variants

Expected

Define a single, consistent model for column accessors.

Acceptance criteria

• Decision made on supported accessor APIs
• Redundant or inconsistent variants are:
  • removed, or
  • aligned to identical behavior
• Naming and usage are consistent across API
• Examples and documentation use the same approach

Motivation

Column access is a core part of the DSL.

Having multiple overlapping APIs creates confusion and increases long-term maintenance cost.

This should be resolved before 1.0 to avoid breaking changes later.

initial comment from @Jolanrensen

Especially in the [Columns Selection DSL](https://github.com/Kotlin/dataframe/pull/372) the library allows the user to create column accessors using either the constructors `column()`, `columnGroup()`, etc. from [constructors.kt](https://github.com/Kotlin/dataframe/blob/master/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/constructors.kt) as well as `col()`, `colGroup()`, ""<>(), ""[""], etc. (now with runtime checks) from the selection DSL [ColumnSelectionDsl.kt](https://github.com/Kotlin/dataframe/blob/continued-columnsselectiondsl/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/ColumnSelectionDsl.kt), [col.kt](https://github.com/Kotlin/dataframe/blob/continued-columnsselectiondsl/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/col.kt), and [colGroup.kt](https://github.com/Kotlin/dataframe/blob/continued-columnsselectiondsl/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/colGroup.kt).

Having both is confusing. We should keep one of the two variants, or have them be exactly the same. Column shortcuts like `""<>()` should be available library-wide too I think, as it's common enough.

Can be handled after https://github.com/Kotlin/dataframe/pull/372