[WIP] Unshim Iceberg reader and delete helpers [fast-ut][reduced-it]

[gerashegalov]

Related to #15821.

Stacked on #15824.

Description

Continue reducing special root-layout treatment for Iceberg by grouping several small, closely related helper refactors:

• Move position-delete field IDs, reader mode types, reader utility methods, and the deletion-vector container into the root-safe iceberg-common module.
• Move delete-filter constants and helper methods into the existing version-specific GpuDeleteFileInfo companion.
• Remove six obsolete Iceberg class entries from dist/unshimmed-common-from-single-shim.txt.

The logic is unchanged. Root-safe helpers are now emitted directly at the jar root, while helpers tied to Iceberg implementation classes remain in the selected parallel world.

AI assistance: Codex assisted with implementation and validation. The author reviewed the complete diff and this description before the PR was updated.

TODO before marking ready for review:

• Land the prerequisite draft PR [WIP] Extract Iceberg schema accessors to root-safe module [fast-ut][reduced-it] #15824.
• Refresh this stacked branch after the prerequisite lands.
• Address CI and review feedback.

Validation:

• Built iceberg-common for Spark 3.3.0, 3.5.7, 4.0.0, and 4.1.2.
• Built the Iceberg 1.6, 1.9, and 1.10 consumer modules.
• Ran GpuDeleteFilterSuite: 6 tests passed.
• Ran GpuPostProcessorSuite: 18 tests passed.
• Ran IcebergDeletionVectorSuite: 5 tests passed.
• Verified the new helpers are present in the root-safe module artifacts and the removed companions are absent from the version artifacts.

Performance: Not required because this change only relocates helpers or changes their owning companion without changing algorithms, data flow, or executed operations.

Checklists

Documentation

• Updated for new or modified user-facing features or behaviors
• No user-facing change

Testing

• Added or modified tests to cover new code paths
• Covered by existing tests
  (GpuDeleteFilterSuite, GpuPostProcessorSuite, and IcebergDeletionVectorSuite.)
• Not required

Performance

• Tests ran and results are added in the PR description
• Issue filed with a link in the PR description
• Not required