feat(builder): add SrcAllowingUnboundedOverdraft combinator

[paul-nicolas]

Adds the allowing unbounded overdraft source combinator on top of #163, required by transaction-plane EN-1271. Base is #163's branch.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.35%. Comparing base (4eb046c) to head (dc06268).

Additional details and impacted files

@@                   Coverage Diff                   @@
##           feat/script-builder     #164      +/-   ##
=======================================================
+ Coverage                68.31%   68.35%   +0.04%     
=======================================================
  Files                       56       56              
  Lines                     5391     5395       +4     
=======================================================
+ Hits                      3683     3688       +5     
  Misses                    1495     1495              
+ Partials                   213      212       -1     

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[NumaryBot]

🛑 Changes requested — automated review

The new exported combinator can generate unparsable Numscript for valid Go calls that pass non-account sources. This should be corrected before considering the patch fully correct.

[NumaryBot]

🟠 [major] Restrict overdraft combinator to account sources

When callers wrap any composite source such as SrcInorder(...) or SrcCapped(...), this exported API still appends allowing unbounded overdraft, producing scripts like { ... } allowing unbounded overdraft or max ... from ... allowing unbounded overdraft that the Numscript grammar cannot parse. The clause is only valid on the account/color source form, so the builder should avoid accepting arbitrary Source values here or otherwise prevent invalid compositions.