Finalize Iceberg root and shim class placement [reduced-it]

[gerashegalov]

Related to #15821.

Stacked on #15841.

Description

This PR completes the Iceberg class-placement milestone:

• promote conditional root-safe module classes from the newest selected Spark classifier that provides each class
• move five conditional Iceberg helpers into iceberg-common
• retain all remaining Iceberg implementation classes in their Spark-specific directories instead of spark-shared
• remove all seven remaining Iceberg entries from unshimmed-common-from-single-shim.txt
• apply the same conditional-root selection to the standard and fast distribution assemblers

After this change, assembled multi-shim distribution JARs contain no Iceberg classes under spark-shared. The shared, root-safe Iceberg helpers are at the JAR root, while Spark- and Iceberg-version-specific implementations remain under sparkXYZ.

There are no user-facing configuration or API changes.

This PR remains a draft while stacked:

• Move independent Iceberg helpers to root-safe module [reduced-it] #15841 is merged
• main is up-merged and CI is rerun

Validation:

• clean Scala 2.12 multi-shim dist build: 334,344,353,359
• clean Scala 2.13 multi-shim dist build: 353,359,404,413,420
• package-private-access audit passed for 11 callers across all selected real Iceberg runtime worlds
• final JAR inspection found 50 Iceberg class files at root, zero under spark-shared, and the remaining classes under Spark-specific directories
• verified that conditional root classes have no copies under the selected shim directories
• verified newest-provider precedence: general helpers came from Spark 4.1.3 and GpuBaseReader came from its newest applicable classifier, Spark 3.5.3
• Spark 4.1.3 focused Iceberg unit tests: 5 passed
• Spark 3.5.3/Scala 2.13 GPU integration smoke with Iceberg loaded through extraClassPath: 6 passed
• fast direct multi-shim assembler completed successfully and produced the same Iceberg placement
• shell, Python syntax, Scala 2.13 POM synchronization, and whitespace checks passed

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
  (IcebergDeletionVectorSuite and the Iceberg read, append, and no-op-write integration smoke tests)
• Not required

Performance

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

Performance testing is not required because this changes build-time source selection and JAR placement without modifying runtime execution logic.

This change and its validation were prepared with AI assistance and reviewed by a human before publication.