feat(useContractorSignatureForm): add contractor W-9 signature form hook

[serikjensen]

Summary

Adds the headless useContractorSignatureForm hook — the data/validation/field layer for signing a contractor document (W-9). This is the first of two PRs; the Contractor.SignatureForm component that consumes it follows in a stacked PR.

• Static field surface (form.Fields) with presence-based gating: core fields are always present, API-variable fields are exposed only when the API returns them (undefined otherwise) — a defensive guard against API divergence.
• Collapses the seven federal tax-classification checkboxes into a single required radio group with conditional LLC-code and "Other" sub-fields, mapping the selection back to the W-9 wire format on submit.
• SSN/EIN format validation with an N/A sentinel; omits untouched redacted TINs on submit so the backend falls back to the real profile value.
• Consolidates the duplicate EIN regex into a shared EIN_REGEX (helpers/validations.ts), now also consumed by contractorDetailsSchema.
• Adds a placeholder field-metadata property for masked-value display.
• onSubmit returns HookSubmitResult<DocumentSigned>, so DocumentSigned is re-exported through the public API models.

Test plan

• npm run test -- --run src/components/Contractor/Documents/SignatureForm/useContractorSignatureForm (43 passing)
• npx tsc --noEmit
• npm run derive (API report + docs regenerated, 0 errors)
• Component wiring verified in the stacked follow-up PR

Separated this into smaller PRs, but worked on both together and verified this hook with the UI as follows:

Screen.Recording.2026-06-30.at.4.52.45.PM.mov

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