Add indexed_map and indexed_map_ref methods to Grid

[Copilot]

Adds indexed_map methods that transform grid elements with access to their (row, col) coordinates.

Changes

• indexed_map<U, F>(self, f: F) -> Grid<U> - Consuming variant that maps over (index, value) pairs via indexed_into_iter()
• indexed_map_ref<U, F>(&self, f: F) -> Grid<U> - Borrowing variant via indexed_iter()
• Both preserve grid dimensions and storage order (RowMajor/ColumnMajor)
• Marked with #[must_use] following the map/map_ref pattern

Usage

let grid: Grid<i32> = grid![[1,2][3,4]];

// Filter based on position
let lower_triangle: Grid<Option<i32>> = 
    grid.indexed_map(|(row, col), v| (row > col).then_some(v));
// Result: [[None, None], [Some(3), None]]

// Add coordinates to values
let result = grid.indexed_map_ref(|(row, col), v| v + row as i32 + col as i32);
// Result: [[1, 3], [4, 6]]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[feature request] Indexed map</issue_title>
<issue_description>Sometimes there is a need to use indexes inside the mapping function. For now there seem to be no convenient way to do it.

impl<T> Grid<T>
{
  fn indexed_map<U, F>(self, f: F) -> Grid<U>
  where F: Fn((usize, usize), T) -> U
  { todo!() }
}

and usage example

let grid: Grid<i32> = ...;
let grid2: Grid<Option<i32>> = grid.indexed_map(|(x, y), v| (x > y).then(v));

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [feature request] Indexed map #71

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