Add an unactivated v2 household prompt contract for issue 165

[MaxGhenis]

Summary

Related to #165. This is an unactivated household prompt-contract slice, not
completion of issue 165 or a new benchmark board. The published v1 builder
omits integer inputs and uses a generic disability label. The new, explicitly
opt-in policybench.prompt_contract_v2 renderer exposes supported integer SSDI
duration, separate disability gates, and absent/unknown provenance while leaving
the published v1 and board files unchanged.

The renderer freezes the take-up/weekly-hours conventions, labels employer-paid
premiums, preserves numeric precision, rejects incompatible supported values,
and marks unsupported scalar fields without guessing their meaning or units.
It pins version 2.0.0 to the new module's source SHA-256 and a complete rendering
of the existing scenario 074 fixture. It imports no engine or v1 prompt helper,
performs no calculation, and is not imported by an evaluator.

Validation

• 76 dedicated tests pass, including all 100 frozen public household fixtures,
  source/golden identity, program labels, unknown provenance, and invalid inputs.
• 20 existing v1 prompt tests pass. Both suites run under an artifact harness
  that refuses network access and actual PolicyEngine package imports.
• New-module statement coverage: 96% (208/216 statements).
• Installed Python 3.10.19 and 3.12.14 reproduce all 100 fixture renderings and
  the exact golden; pytest ran on installed Python 3.14.0.
• Repository-wide Ruff lint and format checks pass.
• All 286 preexisting files other than the authorized Vercel configuration match
  the fetched base by SHA-256. Existing Vercel settings are preserved.

Scope and gates

Four new paths contain the v2 module, its tests, its golden fixture, and
dedicated documentation. The sole additional change adds an exact-branch
Vercel deployment exclusion to app/vercel.json. No cost ledger, cost
schema/reporting, supervisor,
v1 contract, reference, board, or migration file changes are included. Local
contract validation made no model calls, household microsimulations, reference
regeneration, or publication. Normal existing CI is separately authorized.

The new module is a partial fact contract. It does not define requested outputs,
answer schemas, or marital-unit mappings. Unknown values/provenance and
unsupported inputs remain blockers for later evaluation. Suitable data and real
per-fact provenance, engine assumption checks, and the standing reference
sensitivity/exclusion gate remain future work. All existing approvals remain
required. An independent host review of 0c9bbfc found one P3
single-line-validation defect (PB-CONTRACT-001). This head fixes it and adds
10 regression cases; the contract identity/golden header is intentionally
updated and the fixture body is unchanged.

app/vercel.json disables automatic deployment only for
codex/issue165-prompt-contract-v2. The live project root is app. The
configuration validates against the official Vercel schema; there are no
matching true rules. main, the cost branch, and other branches retain the
default deployment behavior. No remote project settings changed.

All normal hosted checks passed for 94c474f: lint, test, app, and paper.
No Vercel deployment was created for the exact branch/head. This draft remains
unactivated and unmerged; all existing review and CI gates remain required.