Extend full_stack fuzz test to cover splicing

[wpaulino]

This includes a new seed that covers the splice flow to completion
(exchanging `splice_locked` and promoting the `FundingScope`).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.08%. Comparing base (fcc3d33) to head (b52b069).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4361      +/-   ##
==========================================
- Coverage   86.09%   86.08%   -0.01%     
==========================================
  Files         156      156              
  Lines      102804   102804              
  Branches   102804   102804              
==========================================
- Hits        88508    88503       -5     
- Misses      11787    11794       +7     
+ Partials     2509     2507       -2     

Flag	Coverage Δ
tests	86.08% <ø> (-0.01%)	⬇️

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[TheBlueMatt]

Hell yea. Trivial (ignoring the stuff in 4336 that already landed).

The merge-base changed after approval.

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[TheBlueMatt]

Hell yea. Trivial (ignoring the stuff in 4336 that already landed).

[TheBlueMatt]

weird, github wont let me land this, i guess it does need rebase.

The merge-base changed after approval.