Disable multithreading when seq_ends is passed as a tuple

[gdalle]

This improves performance and type-stability for single-sequence routines by removing the overhead of @threads.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.07%. Comparing base (d9e4a5d) to head (603def8).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inference/forward.jl	90.90%	1 Missing ⚠️
src/inference/forward_backward.jl	88.88%	1 Missing ⚠️
src/inference/viterbi.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
+ Coverage   89.77%   90.07%   +0.30%     
==========================================
  Files          17       17              
  Lines         489      504      +15     
==========================================
+ Hits          439      454      +15     
  Misses         50       50              

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