Add Colorado alternative minimum tax

[MaxGhenis]

Summary

Implements Colorado alternative minimum tax per C.R.S. § 39-22-105:

• Rate: 3.47% of Colorado AMTI
• CO AMTI = Federal AMTI less exemption + CO additions - CO subtractions
• CO AMT = max(0, tentative minimum tax - normal tax)

New variables:

• co_amt - Colorado alternative minimum tax
• co_tentative_minimum_tax - Tentative minimum tax (3.47% × CO AMTI)
• co_alternative_minimum_taxable_income - Colorado AMTI

Closes #2708

Test plan

• Unit tests for all 3 new variables (6 tests total)
• Verified CO AMT is included in co_income_tax

TAXSIM Validation

Validated CO income tax against TAXSIM 35 for 2023:

Income	PE (before credits)	TAXSIM	Difference
$50,000	$1,590.60	$1,573.03	1.1%
$100,000	$3,790.60	$3,815.31	0.7%
$250,000	$10,390.60	$10,549.22	1.5%
$500,000	$21,472.00	$21,747.57	1.3%

Notes:

• PolicyEngine matches TAXSIM within 1-2% before refundable credits
• Final tax differs due to $800 CO sales tax refund (TABOR) that TAXSIM may not model
• CO AMT rarely triggers with standard income because the AMT rate (3.47%) is lower than the regular rate (4.4%)
• AMT only triggers when CO AMTI exceeds CO taxable income by ~27%, which typically requires specific AMT preference items (ISOs, private activity bond interest) not available in TAXSIM
• TAXSIM doesn't separately report state AMT

Sources:

• Colorado DR 0104AMT
• C.R.S. § 39-22-105

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8272414) to head (8a96cbc).
⚠️ Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #6862   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         4    +3     
  Lines           16        50   +34     
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+ Hits            16        50   +34     

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

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[PavelMakarchuk]

LGTM after the nits

[PavelMakarchuk]

Suggested change

	2021-01-01: 0.0347
	2000-01-01: 0.0347

[PavelMakarchuk]

active voice