Port the UK stochastic layer: take-up anchoring, identity-keyed draws, LHA/BRMA, provenance flags #680

Workstream E4 of the UK-migration master epic #665; child of #145.

Scope

• Port the reported-receipt take-up anchoring (utils/takeup.py: reported recipients take up with certainty; the residual target count fills across non-reporters at an adjusted rate), applied at benunit level to child benefit, pension credit, and universal credit as the incumbent does.
• The 11 take-up + 3 stochastic parameter files land as committed build/uk/ JSON resources with citations, referenced by stage manifests — never Python literals.
• Identity-keyed draws: a UK twin of the US _stable_draws (blake2b over entity id + seed + per-stage salt) replaces the incumbent's sequential rng streams. Unit-record draws therefore differ from the incumbent by construction — a standing signed-difference class; parity is weighted-distributional only, which UKDS disclosure rules require anyway. Incumbent seed constants recorded as documentation (brma 0, take-up 100, property 0, has_fuel 42, cgt 0/1, salary sacrifice 2024, student loans 42).
• LHA/BRMA assignment through the declared rules-engine adapter (LHA_category is real model logic; duplicating it invites drift); the BRMA rent table is a pinned resource.
• TV licence ownership/evasion, first-time-buyer, and property-purchase draws; household_is_* provenance flags as first-class columns (the calibration diagnostics auto-discover them).
• UC take-up stays at the incumbent's 0.55 for the parity build (changing it moves every UC column against the frozen register); follow-up on the caseload undercount is tracked against policyengine-uk-data#452 and WS-D.

Acceptance

• Every stochastic assignment has a determinism test: two runs identical, and row-permutation identity-stability (permute the frame, run, un-permute — values identical per entity id).
• Rates are declared stage parameters with citations; BRMA distribution by region × LHA category within tolerance, with the RNG-stream divergence signed.

UK-specificity note (required in the PR)

Take-up program semantics and BRMA geography are UK; the anchored-assignment and identity-keyed-draw operations should be generic (the US already has assign_binary_from_rate / calibrate_binary_assignment). List anything beyond that and why.

Depends on: E3; the E1 adapter.