Split `parquet/src/arrow/arrow_reader/selection/mod.rs` into smaller modules

[haohuaijin]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Follow up to #10141.

parquet/src/arrow/arrow_reader/selection/mod.rs has grown past 2100 lines (plus ~1300 lines in selection/boolean.rs) and now mixes several concerns:

• the RowSelection type and its public API,
• the set-algebra helpers (intersection / union / and_then and the selector-merge algorithms),
• the cursor machinery (RowSelectionCursor, RowSelectionPolicy, RowSelectionStrategy),
• a large test module.

This was noted during review of #10141 but deferred to keep that PR's diff reviewable.

Describe the solution you'd like

A mechanical, no-functional-change reorganization, e.g.:

• selection/mod.rs — RowSelection, RowSelector, public API surface
• selection/algebra.rs — intersection/union/and_then and merge helpers
• selection/cursor.rs — cursor and policy/strategy types
• selection/boolean.rs — (existing) mask-specific implementation
• tests moved next to the code they cover

The public API (parquet::arrow::arrow_reader::{RowSelection, RowSelector, RowSelectionCursor, RowSelectionPolicy, RowSelectionIter, MaskRunIter}) must be unchanged.

Describe alternatives you've considered

Leaving the file as is; it works but is increasingly hard to navigate and review.