Remove the `copy` parameter from DataFrame.to_pylibcudf(),
Series.to_pylibcudf(), and Index.to_pylibcudf() since it was never
implemented and always raised NotImplementedError.

Updated docstrings to clarify that these methods always perform zero-copy
operations and return views of the existing data.

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Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Add a deep copy method for pylibcudf.Table that creates a new table with
deep copies of all columns. The implementation follows the same pattern as
Column.copy(), accepting optional stream and memory resource parameters.

Changes:
- Added Table.copy() method in table.pyx
- Added method declaration in table.pxd
- Added type stub in table.pyi
- Added test_table_copy() to verify deep copy behavior

The copy method iterates over all columns and calls copy() on each,
ensuring complete independence between the original and copied tables.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

With the buffer unwrapping changes in to_pylibcudf(), buffers are no longer
truly "exposed" during construction. The pylibcudf source objects maintain
references to the original buffer owners, ensuring data stays alive even if
cudf's wrapper is deleted during a spill.

Changes:
- Remove exposed parameter from BufferOwner.__init__() and from_device_memory()
- Remove exposed parameter from SpillableBufferOwner.from_device_memory()
- Remove exposed parameter from as_buffer() utility function
- Remove exposed parameter from all Column type constructors
- Remove data_ptr_exposed parameter from ColumnBase.from_pylibcudf()
- Remove data_ptr_exposed parameter from ColumnBase.from_cuda_array_interface()
- Remove data_ptr_exposed=True from high-level API call sites

Buffers are now only marked as exposed through implicit detection when:
- Raw ptr property is accessed outside an access context
- __cuda_array_interface__["data"][0] is accessed
- scope="external" is used in an access context

This ensures correct spillable buffer behavior and copy-on-write semantics.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

…_args

Replaced individual __init__ overrides in column subclasses with a unified
_validate_args classmethod pattern. This eliminates code duplication and
centralizes validation logic.

Changes:
- Added ColumnBase._validate_args() classmethod for plc_column/dtype validation
- Removed __init__ overrides from NumericalColumn, DatetimeColumn, TimeDeltaColumn,
  DecimalBaseColumn, ListColumn, StructColumn, and IntervalColumn
- StringColumn retains minimal __init__ for instance attribute initialization
- Removed deprecated _validate_dtype_instance methods
- Each column type now overrides _validate_args() for type-specific validation

Benefits:
- Reduces code duplication across 8+ column types
- Centralizes validation logic in one place
- Maintains all existing validation behavior
- Easier to maintain and extend

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Replaced manual caching of _start_offset and _end_offset with
@functools.cached_property, eliminating the need for a custom __init__
method in StringColumn.

Changes:
- Converted start_offset and end_offset from manual @Property with
  conditional caching to @cached_property
- Removed __init__ override that initialized _start_offset and _end_offset
- Removed class-level type annotations for _start_offset and _end_offset
- Simplified offset computation logic without manual cache checks

Benefits:
- Eliminates the last __init__ override in column subclasses
- Reduces boilerplate code for caching
- More Pythonic use of standard library features
- Automatic caching behavior without manual state management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

- Wrap transpose() with access_columns() to prevent buffers from being
  incorrectly marked as exposed during pylibcudf calls
- Update __cuda_array_interface__ to use scope="external" to properly
  mark buffers as exposed when external code accesses the pointer
- Update test_df_transpose to expect buffers NOT to be exposed after
  normal DataFrame operations (only when explicitly accessed)
- Update test_series_zero_copy_cow_on expectations: shallow copies
  share memory with source, so both change when external array changes
- Remove exposed parameter from as_buffer() call in test_get_ptr

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>