Unify column accessor API around col/colGroup and deprecate constructor aliases

[Copilot]

Column accessor creation currently overlaps across constructor-style APIs (column, columnGroup) and DSL APIs (col, colGroup), with inconsistent guidance and discoverability. This PR aligns the model by making col/colGroup the canonical accessors while keeping constructor variants as compatibility aliases.

• API unification (canonical accessors)

  • Deprecated constructor-style overloads in constructors.kt:
    • column(name|path) and ColumnGroupReference.column(name|path)
    • columnGroup(name|path) and ColumnGroupReference.columnGroup(name|path)
  • Added ReplaceWith(...) hints to guide migration to col(...) / colGroup(...).

• Behavior alignment guarantees

  • Added focused tests asserting constructor aliases and DSL accessors resolve identically for:
    • top-level col/column
    • top-level colGroup/columnGroup

• Documentation consistency

  • Updated String API docs to explicitly present col(...) and colGroup(...) as canonical.
  • Clarified column(...) and columnGroup(...) are legacy aliases retained for backward compatibility.

// before (still supported, now deprecated)
column<Int>("age")
columnGroup("name")

// after (canonical)
col<Int>("age")
colGroup("name")