QBI v2 engine: qualification derivations + host-conditioned SSTB (fail-closed)

[MaxGhenis]

Summary

Builds the v2 machinery on top of the #532 fidelity port, per the evidence-anchored spec on #530. v1 remains the production default; v2 fails closed until crosswalk content and published priors land.

• qbi_assumptions_v2.json + loader: per-source qualification as derived | prior with a rationale string each; an sstb_classification block (crosswalk resource, ambiguous prior, passive pass-through SSTB priors by AGI band); per-family independent rng streams — removing one family's draws leaves every other family's stream byte-identical (proven by test).
• Qualification derivations: where law determines the flag, no rng is consumed (resolves the Should *_would_be_qualified columns be derivations rather than QRF targets? #416 forests-over-coin-flips question on the v2 path); the QRF-target exclusion seam sits behind the version gate.
• Host-conditioned SSTB: a post-QRF transform classifies business_is_sstb from host occupation through the versioned crosswalk (clear/non-SSTB deterministic; ambiguous → seeded Bernoulli), passive pass-through income draws from AGI-band priors, then the SE split and SSTB W-2/UBIA pools reroute and the mutually-exclusive qualification routes re-derive — ending in a state that satisfies every existing reconciliation invariant.
• Fail-closed placeholder: sstb_crosswalk_placeholder.json ships with "status": "placeholder"; running v2 against it outside tests raises.
• Industry seam: the frozen census_cps columns carry AGI and PEIOOCC but no detailed industry, so the spine is occupation-first with an explicit industry_column: null seam and a synthetic industry-primary test for the future wire-up.
• Includes the target-parity feed pin refresh (stale since Add JCT tax-expenditure target references: ALD components + CDCC (supersedes #514) #519), filed standalone as Refresh target-parity feed pin to buildn v9.4 (stale since #519) #533 — it drops out of this diff when Refresh target-parity feed pin to buildn v9.4 (stale since #519) #533 merges.

Verification (supervisor, independent)

• Full workspace suite with restricted artifacts: 3,247 passed, 0 failed; ruff clean.
• Wheel packages qbi_assumptions_v2.json and sstb_crosswalk_placeholder.json; both declared in country_package.json.
• v1 golden tests untouched and passing.

Awaiting content (v2 stays fail-closed until then)

• SSTB crosswalk content (census occupation → §1.199A-5 category) — research in flight
• Passive pass-through SSTB priors by AGI (published source) — research in flight
• SCF employer-structure W-2/UBIA conditionals (v3)

Refs #530

🤖 Generated with Claude Code

[MaxGhenis]

Note on CI: the PR gate triggers on base=main only, so no checks run while this is stacked on qbi-port-530; they fire automatically when #532 merges and the base flips. Verification of record meanwhile is the supervisor's independent local run in the PR body: full workspace suite 3,247 passed / 0 failed with restricted artifacts, ruff clean, wheel-gate resources verified.