FIX: Restrict carpet plot crown mask to BOLD coverage

[lobennett]

Closes #3359.

Changes proposed in this pull request

• Add _clip_mask_to_coverage helper in fmriprep/workflows/bold/confounds.py that
  restricts a mask to voxels with non-zero BOLD variance.
• Add crown_clip Function node to init_bold_confs_wf and route it between
  subtract_mask and the three downstream consumers (outputnode.crown_mask,
  crowncompcor.mask_files, mrg_compcor.in3).

Why

When the brain mask is dilated past the BOLD coverage region, the resulting
crown includes voxels with no time-series content. These zero-variance rows are
grouped together by the carpet plot's row-sort and surface as a flat gray band:

Before	After

This is a visualization-only change; edge_comp_* columns in
desc-confounds_timeseries.tsv are unaffected (zero-variance voxels contribute
nothing to PCA). See the issue thread for the verification.

[codecov]

Codecov Report

❌ Patch coverage is 15.38462% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.41%. Comparing base (acdb0fd) to head (1b82ebc).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
fmriprep/workflows/bold/confounds.py	15.38%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3642      +/-   ##
==========================================
- Coverage   74.57%   74.41%   -0.16%     
==========================================
  Files          62       62              
  Lines        4975     4988      +13     
  Branches      637      637              
==========================================
+ Hits         3710     3712       +2     
- Misses       1131     1142      +11     
  Partials      134      134              

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[effigies]

LGTM, thanks.