Net-new UK stages: UC deduction attributes, bus fares, WAS debt

[juaristi22]

Workstream E9 of the UK-migration master epic #665; child of #145. These stages are net-new — no eFRS parity reference exists for their outputs. Acceptance is against their own declared sources; new columns join the export surface through reviewed allowed_extra_columns edits and are recorded as signed additions, never smuggled into the parity register.

Scope

• UC deduction attributes (policyengine-uk-data#450): a new imputation, not a port — latent deduction rate and type combination imputed at build time from DWP pre-FRR distributions staged as pinned artifacts. Estimation moves out of the model-side fallback (policyengine-uk#1815).
• Bus fares: re-authored under the current stage pattern (LCFS fare + ETB subsidy imputations with support clips to the donor's realized range; DfT anchors as declarations). Supersedes draft PR Add UK bus-spending imputation plan and DfT calibration targets #153, which predates the Frame carrier — salvage the DfT anchor values, the London/England split, and citations; the calibration-target half of that PR belongs to WS-C/D.
• WAS debt (policyengine-uk-data#426): mortgage_debt and consumer_debt for net_wealth, if not already taken in the WAS wealth child.

Acceptance

• Declaration-lockstep tests; support evidence per output; determinism per the WS-E harness.
• Export-surface additions reviewed; outputs recorded as signed additions to the parity known-gaps register.

UK-specificity note (required in the PR)

DWP deduction taxonomies and DfT anchors are UK; the imputation and clip operations should be generic. List anything beyond that and why.

Depends on: E1 + the relevant predictor stages (E5/E6). Parallelizable with E7/E8.