r.coin: Add JSON output support

[sumitchintanwar]

This PR adds JSON output support for r.coin module.

Below is an example of JSON Output for r.coin:

{
  "project": "nc_spm_full_v2beta1",
  "created": "2026-03-17T16:38:11Z",
  "region": {
    "north": 228500,
    "south": 215000,
    "east": 645000,
    "west": 630000,
    "ewres": 30,
    "nsres": 30
  },
  "mask": null,
  "maps": [
    {"name": "geology", "title": "Geology", "type": "raster"},
    {"name": "soils",   "title": "Soils",   "type": "raster"}
  ],
  "unit": "square_miles",
  "row_cats": [18683, 19401, 19447],
  "col_cats": [217, 262, 270],
  "matrix": [
    [0.503168, 0.000000, 0.000000],
    [0.000000, 0.023977, 0.000000],
    [0.326990, 0.099730, 0.000000]
  ],
  "row_totals": [0.503168, 0.023977, 0.426720],
  "row_totals_without_zero": [0.503168, 0.023977, 0.426720],
  "col_totals": [0.830158, 0.123707, 0.000000],
  "col_totals_without_zero": [0.830158, 0.123707, 0.000000],
  "totals": {
    "value": 0.953865,
    "value_without_zero": 0.953865
  }
}

Results: All tests pass with no issues on the nc_spm_full_v2beta1 dataset

[petrasovaa]

Let's first discuss the JSON structure... Here is a suggestions that mimicks the plain ascii:

  {
      "project": "spearfish",
      "created": "2025-06-01T...",
      "region": {
          "north": 4928000.00,
          "south": 4914000.00,
          "east": 609000.00,
          "west": 590000.00,
          "ewres": 30,
          "nsres": 30
      },
      "mask": null,
      "maps": [
          {"name": "owner", "title": "Ownership", "type": "raster"},
          {"name": "roads", "title": "Roads", "type": "raster"}
      ],
      "unit": "square miles",
      "row_cats": [0, 1, 2, 3, 4, 5],
      "col_cats": [1, 2],
      "matrix": [
          [50.63, 37.49],
          [ 1.53,  0.68],
          [ 2.92,  0.72],
          [ 3.97,  2.57],
          [ 0.65,  1.36],
          [ 0.19,  0.00]
      ],
      "row_totals": [88.12, 2.21, 3.64, 6.54, 2.00, 0.19],
      "row_totals_without_zero": [88.12, 2.21, 3.64, 6.54, 2.00, 0.19],
      "col_totals": [59.90, 42.80],
      "col_totals_without_zero": [9.27, 5.32],
      "totals": {"value": 102.70, "value_without_zero": 14.58}
  }

Note there are very similar tools - r.kappa and r.report, r.report has nested schema while r.kappa provide a value matrix. It seems r.coin is more similar to r.kappa in this regard, so hence I suggest the JSON above.

[sumitchintanwar]

Noted on the JSON format. Will come up with a fix soon.

[sumitchintanwar]

@petrasovaa I have changed the json output format and added some tests for the values.