Add alpha transparency parameter to 3D heatmaps

[AdityaGupta716]

Closes #101

Summary

Adds an alpha parameter to Heatmap.__init__() so users can control
brain region transparency in 3D format directly, without manually
accessing brainrender actors.

Usage

Global alpha (all regions):

bgh.Heatmap(values, ..., format="3D", alpha=0.5)

Per-region alpha:

bgh.Heatmap(values, ..., format="3D", alpha={"HIP": 0.3, "TH": 0.8})

Changes

• brainglobe_heatmap/heatmaps.py — new alpha parameter with validation
• tests/test_unit/test_alpha.py — unit and integration tests
• examples/heatmap_3d_alpha.py — new example

Notes

• None by default — fully backwards compatible
• Has no effect in 2D format
• Values validated at init time with clear error messages

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.91%. Comparing base (0fea8e1) to head (54830a1).

Files with missing lines	Patch %	Lines
brainglobe_heatmap/heatmaps.py	53.33%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   69.63%   68.91%   -0.72%     
==========================================
  Files           5        5              
  Lines         326      341      +15     
==========================================
+ Hits          227      235       +8     
- Misses         99      106       +7     

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[AdityaGupta716]

hi @adamltyson plz review whenever ur free , let me know if there are any changes u would like !