refactor: extract common graph construction patterns into graph.rs

[Copilot]

Several puzzle files under cspuz_rs_puzzles/src/puzzles/ were constructing identical or near-identical Graph structures inline. This PR factors out the recurring patterns into reusable functions in cspuz_rs/src/graph.rs.

New functions

• graph_8_neighbors(shape) — h×w graph with 8-neighbor (4-directional + diagonal) connectivity.
  Used by: bramble.rs, cocktail.rs, tetrochain_common.rs

• graph_8_neighbors_with_outer_vertex(shape) — same as above, plus a single outer vertex (h*w) connected to all border cells.
  Used by: nurimaze.rs, city_space.rs

• active_edges_acyclic(solver, edges: &BoolGridEdges) — adds a no-cycle constraint on a set of grid edges via the planar dual graph. An identical ~40-line block was copy-pasted verbatim in mintonette.rs and move_common.rs.

• graph_slash_connectivity(shape) / graph_slash_connectivity_with_outer_vertex(shape) — sub-cell connectivity graph for slash/backslash puzzles, splitting each cell into 4 parity-indexed quadrants.
  Used by: slashpack.rs / gokigen.rs

Example: before → after

// Before (repeated in bramble.rs, cocktail.rs, tetrochain_common.rs)
let mut g = graph::Graph::new(h * w);
for y in 0..h {
    for x in 0..w {
        if y < h - 1 { g.add_edge(y * w + x, (y + 1) * w + x); }
        if x < w - 1 { g.add_edge(y * w + x, y * w + x + 1); }
        if y < h - 1 && x < w - 1 { g.add_edge(y * w + x, (y + 1) * w + x + 1); }
        if y < h - 1 && x > 0     { g.add_edge(y * w + x, (y + 1) * w + x - 1); }
    }
}
graph::active_vertices_connected(&mut solver, is_black.flatten(), &g);

// After
let g = graph::graph_8_neighbors((h, w));
graph::active_vertices_connected(&mut solver, is_black.flatten(), &g);

[ReverM]

litherslink has the no acyclic constraint as well