diagnostic: Fix false positive for kwargs used in other kwarg defaults

[aviatesk]

When keyword argument defaults depend on other keyword arguments, JuliaLowering's scope_nest creates :local bindings in let blocks instead of :argument bindings. This caused the unused argument analysis to miss the connection between the :argument binding in the body method and the :local bindings where the keyword is actually used in default expressions.

Three changes fix this:

1. Include :argument bindings in same_location_bindings unconditionally (not just for @generated functions), so they get unified with :local bindings at the same source location.
2. Extend skip_arguments in compute_binding_occurrences! to also skip :local bindings in self/kwsorter calls, preventing internal call machinery from being counted as genuine usage.
3. Add has_matching_argument_binding check in analyze_unused_bindings! to suppress duplicate :local diagnostics when a corresponding :argument binding exists at the same location.

Closes #592

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.87%. Comparing base (00f46f2) to head (4ed285b).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/diagnostic.jl	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #593      +/-   ##
==========================================
- Coverage   67.88%   67.87%   -0.01%     
==========================================
  Files          51       51              
  Lines        8295     8303       +8     
==========================================
+ Hits         5631     5636       +5     
- Misses       2664     2667       +3     

Flag	Coverage Δ
JETLS.jl	67.87% <78.94%> (-0.01%)	⬇️

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