Auto-select CSI BAM index for long-chromosome genomes

[etal]

Fixes #817.

ensure_bam_index previously built and recognized only BAI indexes. The
BAI format cannot address reference contigs longer than 2**29 bp (about
537 Mb), so BAM files from long-chromosome genomes — wheat, barley
(chr2H ~665 Mb), salamander — failed to index entirely:

[E::hts_idx_check_range] Region X..Y cannot be stored in a bai index. Try using a csi index

CNVkit now reads the BAM header and, when any reference contig exceeds the
BAI limit, builds a CSI index (samtools index -c) automatically. Typical
human references (hg19/hg38, longest contig ~250 Mb) are unaffected: they
still get a .bai, byte-for-byte as before. An existing .csi is
discovered and reused just like .bai, and a stale .bai is removed after
a CSI is built so it cannot shadow the new index.

Tests

New test/test_samutil.py:

• short contig → BAI is built and returned;
• contig > 2**29 → a CSI index is built, returned, and usable (fetch and
  idxstats work through a read positioned beyond the BAI coordinate limit);
• contig of exactly 2**29 → BAI (verifies the strict threshold);
• a fresh .csi is reused without rebuilding;
• a leftover .bai is removed once a CSI is built.

Existing coverage and batch tests pass unchanged, confirming the BAI
path is unregressed.

Clinical impact

None. No change to numerical output or output file formats (.cnr,
.cns, .cnn, SEG, VCF) for existing workflows; the BAI code path is
unchanged. This only enables genomes that previously could not be indexed
at all.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.41%. Comparing base (42c8ce6) to head (1746322).

Files with missing lines	Patch %	Lines
cnvlib/samutil.py	88.88%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
+ Coverage   70.35%   70.41%   +0.05%     
==========================================
  Files          73       73              
  Lines        7891     7907      +16     
  Branches     1395     1400       +5     
==========================================
+ Hits         5552     5568      +16     
  Misses       1891     1891              
  Partials      448      448              

Flag	Coverage Δ
unittests	70.41% <88.88%> (+0.05%)	⬆️

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