Extract common rendering logic from puzzle solvers into helper functions

[Copilot]

Identified and extracted two common rendering patterns (15+ lines, 5+ files) from puzzle solver implementations.

Changes

New helpers in board.rs:

• add_grid_edges() - renders edge-based puzzles (Slitherlink, Litherslink, Crosswall)
• add_block_dot_answer() - renders boolean answer grids (Norinori, LITS, Cocktail, etc.)

Refactored files: 16 total

• 3 files using edge grid pattern (24 lines → 1 line each)
• 13 files using block/dot pattern (16 lines → 1 line each)

Example

Before:

if let Some(is_black) = &ans {
    for y in 0..height {
        for x in 0..width {
            if let Some(b) = is_black[y][x] {
                board.push(Item::cell(
                    y, x, "green",
                    if b { ItemKind::Block } else { ItemKind::Dot },
                ));
            }
        }
    }
}

After:

if let Some(is_black) = &ans {
    board.add_block_dot_answer(is_black, "green");
}

Net: -264 lines of duplicated code

Original prompt

In cspuz_solver_backend/src/puzzle, several files may share a common logic. Identify such logics and factor out them into a helper function.
Criteria: 15 lines or more of common logic, used in 5 or more files
Do not modify test expectations.

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