fix(Lists): unify indent and outdent for staircase selections

[makhnatkin]

This PR reworks list indentation around a shared block planner for complex multi-item list selections.

Instead of treating the whole selection as a single blockRange, the new logic plans contiguous sibling blocks per list level and applies the same model to both Tab / Mod-] and Shift-Tab / Mod-[.

The planner is now described and tested around six explicit cases:

1. A single selected list item.
2. A flat sibling group on one list level.
3. A downward staircase selection into deeper nested items.
4. An upward staircase selection from nested items back to outer siblings.
5. A mixed multi-level selection that requires planner expansion and regrouping.
6. A multi-block command that stays in one transaction and preserves selection on undo.

Behaviorally, this PR keeps block-based movement semantics:

• the unit of movement is a contiguous sibling block, not an independently moved row;
• the planner may expand the selected structure when nested tails are implied by the current selection;
• indent and outdent share the same planning model and differ only in execution.

The test suite now covers the planner directly, and the Lists folder includes a short README with an algorithm diagram for future maintenance.

[gravity-ui-bot]

Preview is ready.

[gravity-ui-bot]

Visual Tests Report is ready.

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[gravity-ui]