fix(#103): replace mesh projection with direct atlas volume read

[ArielShadrac]

Closes #103

Problem

The 2D frontal slice was showing only contours with no colours, and
the medio-lateral axis was inverted.

Before (bug #103)

After — correct colours and orientation

Code used to generate the render

```python import brainglobe_heatmap as bgh

dict1 = {
'mlf': 1.04, 'IO': 0.5, 'IP': 0.40, 'DCO': 0.88, 'VCO': 0.17,
'chpl': 0.14, 'IRN': 0.009, 'V4': 0.023, 'V4r': 0.11, 'PRP': 0.012,
'py': 0.38, 'MV': 0.097, 'SUV': 0.118, 'RO': 0.07, 'SPIV': 0.099,
'RPA': 0.467, 'MARN': 0.010, 'MY': 0.353, 'ICB': 0.056, 'SPVO': 0.036,
'das': 0.101, 'CB': 1.094, 'sctd': 1.318, 'NTS': 0.172, 'arb': 0.022,
'x': 0.112, 'sptV': 0.220, 'DN': 0.054, 'PARN': 0.009, 'rust': 0.042,
'sctv': 0.119, 'LING': 0.056, 'AMBd': 0.0, 'NOD': 0.419, 'PGRNd': 0.178,
'PGRNl': 0.025, 'FN': 0.016, 'PRM': 0.351, 'COPY': 0.267, 'PFL': 0.9,
'tspc': 0.020, 'GRN': 0.024, 'ANcr2': 0.280, 'icp': 0.096, 'VeCB': 0.053,
}

bgh.Heatmap(
dict1,
position=11500,
orientation="frontal",
thickness=100,
vmin=min(dict1.values()),
vmax=max(dict1.values()),
format="2D",
).show()

</details>


## Changes
- `_get_slice_from_volume`: reads `atlas.annotation` directly instead
  of projecting 3D meshes, fixing the ML-axis inversion
- `_build_region_masks_bottomup`: bottom-up ancestor traversal fixes
  empty masks for parent regions (e.g. CB) whose pixels are carried
  by leaf sub-regions in the annotation volume
- Single cumulative RGBA canvas replaces repeated `imshow()` calls
- Added `color_mode`, `show_labels`, smooth vectorial contours

## Tests
- Unit tests: `tests/test_unit/test_heatmaps_unit.py`
- Integration tests: `tests/test_integration/test_heatmaps_integration.py`

## How to test
pytest tests/ -v