Implement Colorado EITC branch for missing valid employment SSNs

## Summary
Axiom's Colorado RuleSpec encoding for C.R.S. 39-22-123.5 surfaced a PolicyEngine-US gap in Colorado EITC.

Colorado section 39-22-123.5(2.5) allows a resident individual to claim the state EITC amount that would have been allowed, but for the fact that the taxpayer, spouse, or dependent does not have a Social Security number valid for employment.

PolicyEngine-US currently computes Colorado EITC as:

```python
federal_eitc = tax_unit("eitc", period)
return federal_eitc * p.eitc.match
```

That means if federal EITC is zero because of missing valid employment SSNs, Colorado EITC is also zero, with no alternate state branch.

## Axiom context
RuleSpec legal IDs:

- `us-co:statutes/39/39-22-123.5#missing_valid_ssn_eitc_branch_before_part_year_apportionment`
- `us-co:statutes/39/39-22-123.5#missing_valid_ssn_eitc_branch_refundable_excess`
- `us-co:statutes/39/39-22-123.5#missing_valid_ssn_eitc_branch_excluded_from_public_benefit_income_or_resources`

RuleSpec computes the missing-SSN branch from:

- `federal_eitc_would_be_allowed_but_for_missing_valid_employment_ssn`, and
- `federal_earned_income_tax_credit_would_have_been_allowed_but_for_missing_valid_employment_ssn`.

## Expected
PolicyEngine should model the Colorado missing-valid-employment-SSN branch, likely via new inputs or federal-EITC helper outputs representing the EITC that would have been allowed absent the federal SSN requirement.

## Likely files

- `policyengine_us/variables/gov/states/co/tax/income/credits/eitc/co_eitc.py`
- possibly federal EITC eligibility/amount helpers or new user-facing inputs for the Colorado-specific branch

Start in policyengine_us/variables/gov/states/co/tax/income/credits/eitc/co_eitc.py and inspect how Colorado EITC currently derives from federal EITC. Then trace the federal EITC eligibility and amount helpers named in the issue, along with the RuleSpec legal IDs, to determine the available missing-SSN branch inputs. Done means Colorado EITC reflects that branch while preserving the existing calculation and its apportionment and refundable-income treatment.

Written by the indexing model from the issue text.