Add statute-facing taxpayer ID primitives

[MaxGhenis]

Summary

• add canonical taxpayer_id_type and has_valid_ssn person variables
• make has_tin resolve from explicit inputs or the new taxpayer ID primitive during migration
• update federal CTC, EITC, and other SSN-gated rules to use statute-facing ID concepts instead of overloading ssn_card_type

Why

Current federal rules use two distinct identification concepts:

• broader taxpayer-ID (TIN) rules for filer/dependent eligibility in several places
• stricter valid-SSN rules for child CTC, EITC, and newer SSN-gated provisions

This PR makes that split explicit in policyengine-us, while preserving has_itin as a temporary compatibility alias for existing callers.

Companion data PR: PolicyEngine/policyengine-us-data#715

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.66%. Comparing base (b70c547) to head (e762475).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/household/demographic/person/taxpayer_id_type.py	92.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7974      +/-   ##
===========================================
- Coverage   100.00%   93.66%   -6.34%     
===========================================
  Files            1       10       +9     
  Lines           13      142     +129     
  Branches         0        8       +8     
===========================================
+ Hits            13      133     +120     
- Misses           0        5       +5     
- Partials         0        4       +4     

Flag	Coverage Δ
unittests	93.66% <96.15%> (-6.34%)	⬇️

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