[flobinsa]

fix: share filtered-read predicates across fragments

Before

Filtered-read planning deep-cloned logical predicates for each fragment.
Converting the plan into fragment read tasks cloned them again.

Large predicates, such as long IN lists, multiplied memory usage by the
number of fragments and could cause OOMs.

After

Fragments share immutable logical predicates through Arc<Expr>, including
when creating read tasks. Filter-selection and evaluation behavior remain
unchanged.

Changes

• Allocate shared full and refine predicates once per scan plan.
• Clone Arc references instead of expression trees for each fragment.
• Preserve shared ownership in ScopedFragmentRead.
• Extend the existing plan round-trip test to assert that fragment-plan
  filters share one allocation.

Validation

• Filtered-read suite: 86 passed.
• Full Lance library suite: 4,922 passed, 3 ignored.
• Local planner probe with 37,574 IDs across 64 fragments: peak RSS decreased
  from approximately 409 MB to 66 MB.
• Issued a COUNT(*) on local and finished without OOM

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