Add structural Board→text renderer and integrate it into run_solver

[Copilot]

run_solver previously had no compact text output for solved boards. This change adds a backend-side text renderer that infers rendering from Board structure, focuses on answer data (green items), and returns None for unsupported board/item patterns.

• Board text rendering API

  • Added Board::to_text() -> Option<String> in cspuz_solver_backend/src/board.rs.
  • Rendering is structure-driven (cell/edge placement by coordinate parity), not puzzle-specific branching.
  • Supported answer glyphs are centered on green items (e.g. block/dot/number/square and line/cross-style edges).
  • Unsupported green item kinds, conflicting placements, or invalid coordinates return None.
  • Updated edge rendering to show continuous borders (+---+) for dot-grid boards (e.g. slitherlink), while keeping non-dot-grid boards (e.g. yajilin) on the original spacing-based renderer to avoid malformed output.

• run_solver integration

  • Updated cspuz_solver_backend/src/bin/run_solver.rs to print text output in non-JSON mode when to_text() returns Some(...).
  • Existing --json behavior remains unchanged.

• Coverage for required puzzle families

  • Added focused tests to ensure to_text() is produced for required puzzle URLs (nurikabe, fillomino, slitherlink, yajilin, heyawake, dbchoco, evolmino).
  • Added a negative test verifying unsupported item kinds return None.
  • Added regression tests for:
    • dot-grid continuity (+---+) on slitherlink
    • preventing dot-grid-style + vertex rendering from leaking into yajilin output

// run_solver.rs (non-JSON path)
println!("Answer Status: {}", answer_status);
if let Some(text) = board.to_text() {
    println!("\n{}", text);
}

[semiexp]

@copilot

Answer Status: Unique

    x   x   x   |   x
  -   x   x   x   -   x
    |   x   x   x   |
  x   -   x   -   x   x
    |   |   |   |   |
  -   x   -   x   -   x
    |   |   |   |   x
  x   -   x   -   -   -
    |   x   x   x   |
  x   -   -   -   x   x
    x   x   x   |   |

見づらいです。+---+ のように、境界線が途切れなく見えるようにしてください

[Copilot]

@copilot

Answer Status: Unique

...

対応しました。連続した境界線になるように Board::to_text の描画ロジックを更新し、+---+ 形式で表示されるようにしています（commit: aa377f2）。

Screenshot (run_solver output):

Answer Status: Unique

+---+---+---+---+
|   x   x   x   |
+---+ x +---+ x +
x   |   |   |   |
+---+ x + x + x +
|   x   |   |   |
+ x +---+ x +---+
|   |   x   x   x
+---+ x + x + x +

[semiexp]

@copilot

Answer Status: Unique

    +   +   +   +   +           +   +
  . | . | . x . | . | .   #   . | . | .
+---+ x +---+---+ x +---+   +---+   +---+
  . | . x . | . x . | . x . | .   #   .
    +---+   +   +---+   +---+       +---+
  #   .   #       .       .   #       .
    +---+   +   +---+   +---+   +   +---+
      . x . | . | .   #   . | . x . | .
    +---+---+   + x +   + x +---+---+
  . | . x .       . | . | . x . x .
+---+ x +---+   +---+   +---+---+---+
  . | . x .   #   .   #   . x . x . | .
    +---+---+   +---+   +---+---+   +---+
  #   . x .       .       . x .       .
    +---+---+   +---+   +---+---+   +---+
  . | . x . | . | .   #   . | .   #   .
+---+ x + x + x + x +   + x +       +---+
  . x . | . | . | . x . | .       . | .
+---+---+   +   +---+---+---+   +---+
  . | .       #   . | . x . | . | .   #
    +               +   +   +   +

yajilin を入れたらこうなりましたが、描画が明らかにおかしいです。修正してください

[Copilot]

@copilot

Answer Status: Unique

...

修正しました。+---+ の連続境界レンダリングは DotGrid のみに限定し、Yajilin など非 DotGrid は従来の配置ベース描画に戻しています（commit: 45fa1d0）。

Screenshot (run_solver output, yajilin):

Answer Status: Unique

  . | . | . x . | . | .   #   . | . | .
  -   x   -   -   x   -       -       -
  . | . x . | . x . | . x . | .   #   .
      -           -       -           -
  #   .   #       .       .   #       .
      -           -       -           -
      . x . | . | .   #   . | . x . | .
      -   -       x       x   -   -
  . | . x .       . | . | . x . x .
  -   x   -       -       -   -   -
  . | . x .   #   .   #   . x . x . | .
      -   -       -       -   -       -
  #   . x .       .       . x .       .
      -   -       -       -   -       -
  . | . x . | . | .   #   . | .   #   .
  -   x   x   x   x       x           -
  . x . | . | . | . x . | .       . | .
  -   -           -   -   -       -
  . | .       #   . | . x . | . | .   #