[refine](column) separate mutable subcolumn mutation from read-only traversal

[Mryange]

What problem does this PR solve?

Column wrappers used for_each_subcolumn for both read-only traversal and mutable subcolumn detachment during COW mutation. These two operations have different contracts: read-only paths only need const IColumn&, while COW mutation needs to replace child pointer slots.

Root cause: the old mutable callback accepted IColumn::WrappedPtr&, but several composite columns store strongly typed child wrappers, such as nullable null maps and array/map offsets. That forced ad-hoc move-out and cast-back bridges, and made read-only callers use mutable traversal or const_cast.

This change keeps for_each_subcolumn as a const read-only traversal API and adds mutate_subcolumns() for the COW mutation path. Composite columns now detach children through mutate_subcolumn helpers, and read-only recursive checks use the const traversal directly. A dedicated BE unit test file verifies that mutating exclusive subcolumns does not introduce extra clones, while shared subcolumns are still detached.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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