Port capital gains structure, salary sacrifice, and student loan stages

[juaristi22]

Workstream E8 of the UK-migration master epic #665; child of #145. Order constraint: after the SPI channel (income before capital gains and before salary-sacrifice stage 2).

Capital gains — the adjudicated split (2026-08-14)

The incumbent builds CGT in three moves: (1) incidence by doubling the dataset and running an unseeded torch optimization that splits each household's weight between its has-gains and no-gains copies to match the Advani-Summers incidence table; (2) amounts from a percentile spline (stops at p95); (3) a top tail of 30 donor households per HMRC size band at band-exact weights. The adjudication:

• Port the structure: doubling with household_is_capital_gains_clone + the 780 band donors (household_is_cgt_band_donor) — the population needs support for both states and for large gains, the certified spine's record count assumes it, and calibration needs the support to bind CGT band targets.
• Keep the merged Impute capital gains amounts from the published joint distribution #560 amounts stage (HMRC table-3 joint-distribution redraw — better tail than the spline; signed difference vs the incumbent).
• Drop the torch sigmoid-blend: clones get an equal mass split at importance kind; WS-D's calibration against the chronicle CGT band facts does the blending the torch loop used to do. Signed difference with the named WS-D dependency. (The incumbent's blend loop is unseeded, and torch is not a microcosm-build dependency.)

Salary sacrifice

QRF stage 1 on the asked-subset (reported values kept exactly); the 5.4m headcount conversion as a declared anchor with its OBR provenance (a top_up_to_stock-style operation, seeded).

Student loans

Rule-based cohort assignment (plan by estimated university start year; PLAN_4 never imputed — documented UK-specific) plus below-threshold top-up to SLC liable-to-repay stocks. SLC stocks are chronicle-fact-fed (wave 1 carries the SLC facts) — coordinate with @juaristi22 before wiring any chronicle feed (wave-2 pieces are in flight).

Acceptance

• The pre-clone record-count identity reproduced: (base + 10k SPI) × 2 + 780 donors.
• CGT amounts vs the published surface per Impute capital gains amounts from the published joint distribution #560's embedded tests; the weight-mechanism divergence signed with measured incidence deltas.
• Determinism per the WS-E harness; support gate declared for donor-backed families.

UK-specificity note (required in the PR)

HMRC band structure, SLC plan rules, and the OBR anchor are UK; cloning, donor stacking, and stock top-up should be generic operations. List anything beyond that and why.

Depends on: E7 (income), E5 (student_loan_balance); related: #552, #364, #467.