Add soi_iitax parameter

[martinholmer]

Fixes #3071 by adding a new parameter called soi_iitax that controls
the categorization of the self-employment tax (setax) and the
additional Medicare tax (ptax_amc) as being part of the income tax
or as being part of the payroll tax.

It is important that Tax-Calculator have this flexibility because
grouping setax and ptax_amc as being part of the income tax
(soi_iitax==true) reflects the structure of IRS tax forms and
corresponds to SOI statistics used to construct the TMD input data files.
But categorizing setax and ptax_amc as being payroll taxes
(soi_iitax==false) is the tax-analysis approach used by all federal
tax agencies.

This pull request sets the value of soi_iitax to false in all years
so that Tax-Calculator produces conventional tax-analysis results by
default. Note that this is a change from the way Tax-Calculator has
worked since its beginnings. Concurrently, the code in the TMD
repository has been revised so that the value of soi_iitax is true
and Tax-Calculator produces SOI-compatible income tax statistics.

Also, a test has been added to the test_reforms.py module to confirm
that the new soi_iitax logic produces the correct results described
in issue #3071 for a reform that eliminates the cap on FICA and SECA
taxable earnings.

How does PolicyEngine-US handle this issue?

The following prompt:

❯ Looking at the code in the GitHub PolicyEngine-US repository, does that model
include the SECA tax (that is, the self-employment social security tax) in
the income tax total or in the payroll tax total?

produced this Claude Code response:

⏺ Let me look at the PolicyEngine-US repository to find how SECA tax is
categorized.

⏺ Explore(Find SECA tax categorization in PolicyEngine-US)
⎿  Done (64 tool uses · 58.6k tokens · 4m 59s)
(ctrl+o to expand)

⏺ In PolicyEngine-US, SECA tax is in neither — it's kept as its own separate tax
category.

The model has three distinct tax buckets under
/policyengine_us/variables/gov/irs/tax/:

1. federal_income/ — income tax before refundable credits minus refundable
   credits. Does not include SECA.
2. payroll/ — employee Social Security tax, employee Medicare tax, additional
   Medicare tax, employee state payroll tax. Does not include SECA.
3. self_employment/ — self_employment_social_security_tax +
   self_employment_medicare_tax. Stands alone.

So self_employment_tax is a person-level variable summing the SE Social
Security and SE Medicare components, computed independently from both income
tax and employee payroll tax totals.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3c10be1) to head (8711818).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3072   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines         2721      2721           
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  Hits          2721      2721           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)

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[jdebacker]

Very nice PR. I like how you created the boolean to allow modeling of the IRS/SOI concept and traditional revenue estimates.

I have just one question on some files that are removed. When I'm clear on those changes, I'll merge.

[jdebacker]

Ah - those files being removed are just clean up of the repo. Thanks! Merging,