[Repo Assist] fix: make Frequency.isSubset truly generic (remove hardcoded float key type)

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Summary

Fixes a silent type-constraint bug in Frequency.isSubset.

Root cause: The inner recursive helper issubset had explicit type annotations list<float*int> and Map<float,int>. F# type inference propagated these constraints to the outer function, making isSubset accept only Map<float,int> — despite the outer signature appearing generic (Map<_,int>). Users calling isSubset on a string or int histogram (created via Frequency.createGeneric) would get a confusing type error.

Fix: Replace the hardcoded float type annotations in the inner helper with the generic type variable 'a, matching the intended semantics.

Before:

let isSubset (histA:Map<_,int>) (histB:Map<_,int>) =
    let rec issubset (histA:list<float*int>) (histB:Map<float,int>) = ...

After:

let isSubset (histA:Map<'a,int>) (histB:Map<'a,int>) =
    let rec issubset (histA:list<'a*int>) (histB:Map<'a,int>) = ...
```

## Test Status

6 new tests added covering float, string, int, and empty histogram cases:

```
Passed Distributions.Frequency.isSubset float keys - A is subset of B
Passed Distributions.Frequency.isSubset float keys - A is not subset of B
Passed Distributions.Frequency.isSubset string keys - A is subset of B
Passed Distributions.Frequency.isSubset string keys - A is not subset of B
Passed Distributions.Frequency.isSubset int keys - A is subset of B
Passed Distributions.Frequency.isSubset empty histA is always subset

✅ All 1205 tests passed (1199 pre-existing + 6 new).

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.99%. Comparing base (3533493) to head (e3f79d1).
⚠️ Report is 185 commits behind head on developer.

Additional details and impacted files

@@              Coverage Diff              @@
##           developer     #378      +/-   ##
=============================================
+ Coverage      47.61%   47.99%   +0.37%     
=============================================
  Files            152      135      -17     
  Lines          16765    12930    -3835     
  Branches        2253     1709     -544     
=============================================
- Hits            7983     6206    -1777     
+ Misses          8097     6205    -1892     
+ Partials         685      519     -166     

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