feat(Contractor.Documents): document list, W-9 signing, and signer flow

[serikjensen]

Summary

Adds standalone contractor document review + signing (W-9), developed together so it can be split into focused PRs later.

• useContractorDocumentsList + DocumentsList — lists a contractor's documents; Continue gates on all required docs being signed; emits contractor/documents/view / contractor/documents/done.
• useContractorSignatureForm + SignatureForm — dynamic W-9 form driven by the document's fields[] (state-taxes-style). Synthesizes the 7 classification checkboxes into a single radio group with conditional LLC-code / "Other" sub-fields, maps the selection back to the 0/1 wire format on submit, and reuses the shared DocumentViewer.
• ContractorDocumentSigner — thin robot3 orchestrator composing the two (standalone; not wired into ContractorOnboardingFlow yet, per the dedicated-flow plan).
• Redacted SSN/EIN handling — masked values from the API are no longer seeded as the input value (which round-tripped the mask back on submit). They now seed an empty input, surface the mask as a placeholder, are exempt from required validation, and are omitted from the sign payload unless the contractor types a replacement. (Mirrors the employee hasRedactedValue pattern; adds an optional placeholder to FieldMetadata.)
• Dedicated i18n namespaces (Contractor.DocumentsList, Contractor.SignatureForm), scoped events, MSW W-9 mocks, hook/component/orchestrator tests, and stories.
• sdk-app registry regenerated so the dev app injects contractorId (and prompts for documentUuid on the standalone SignatureForm).

Status / TODO before review

• Failing test: SignatureForm "shows the masked SSN as a placeholder" — getByLabelText('Social Security Number (SSN)') no longer resolves after the redacted-field change; needs the query/label association sorted out.
• Split into focused PRs (list → sign hook → sign component → orchestrator).
• Confirm the sign endpoint's expected behavior for omitted redacted fields against gws-flows.

Test plan

• npm run test -- --run src/components/Contractor/Documents
• Exercise list → sign → back / done in the SDK dev app against a real contractor.

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