UK CGT: interpolate the sub-exempt remainder, anchor the clone incidence, and fence projected entrants (#970)

changelog.d/970-cgt-incidence-anchor.added.md

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+UK spine stage `cgt_incidence_anchor` (31st stage, after `hmrc_cgt_asset_type_spine`): a weights-only structural stage that moves the mass of sub-exempt and loss-making clone households back to their paired originals until the two groups match the Advani-Summers reporter composition at the redrawn liable mass. No cell is written; every clone/original pair and the household total are conserved; liable clones and band donors are untouched. Gated at stage time by `uk_stage_cgt_incidence_anchor_composition`; the H2 parity fixture, the release input coverage manifest, the spine gate pins and the identity-stability tool (pair-sum restoration, anchor recompute in the E8 receipt) move with it (microcosm#970).

changelog.d/970-cgt-sub-aea-remainder.changed.md

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+UK CGT amounts stage (`hmrc_cgt_gains_spine`): gainers beyond the published taxpayer mass now take amounts from the Advani-Summers within-band gains distribution restricted to (0, annual exempt amount], rank-preserving within each total-income band and deterministic, instead of being capped at exactly the exempt amount. The shared Advani-Summers surface (loader, band lookup, degree-1 quantile spline and its inverse) moves to `uk_runtime/advani_summers.py`; the stage receipt carries a `remainder` block the stage-health gate checks (microcosm#970).

changelog.d/970-uk-cgt-projection-entrants-gate.added.md

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+UK calibration seam gate `uk_cgt_projection_entrants` (terminal, release-blocking, absence blocks): projects the calibrated frame's sub-exempt capital gains along the engine's per-capita GDP growth path against the frozen annual exempt amount to a 2030 horizon and fails when the cumulative stock of crossers in any year exceeds the vendored HMRC Table 2.1a taxpayers in the thinnest liable band (73,000 in 2024-25), a plausibility ceiling; the engine's growth path and exempt amount are pinned in the manifest and drift-checked. The seam supplies a `cgt_projection` artifact read from the installed engine (`uk_runtime/cgt_projection.py`), or stated from the manifest pins when no engine is installed (the receipt names the source, and the release certifier refuses a seam part whose fence read the pins, so an engine-free seam evaluates the gate but cannot certify a cut); the UK gate battery digests, the certification calibration-seam digests and their data-shard mirrors move (microcosm#970).

docs/evidence/uk-cgt-970/README.md

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+# Evidence for microcosm#970 (UK CGT sub-exempt gainers)
+
+Aggregate-only extracts of the licensed receipts behind `experiments/970-cgt-sub-exempt-remainder.md`. The
+licensed artifacts themselves (spine and calibrated H5 files, full sidecars, the FRS/SPI/WAS/LCFS/ETB inputs) live
+outside the tree under `data/ukds/acceptance/970-cgt-sub-exempt/` and `runs/uk-623-first-calibrated/`.
+
+- `v20-fence-receipt.json`: the projection fence on the calibrated v20 artifact, directly and as the signed seam
+  receipt of calibrating the v20 spine through the branch (blocked at terminal, no H5).
+- `spine-970-receipts.json`: the rebuilt 31-stage spine's anchor and remainder receipts and spine gate verdicts.
+- `v22-calibration-receipts.json`: the v22 seam verdict, fence details, calibration summary, CGT target errors, the
+  pass-2 score against the incumbent, and the clone/donor split of the remaining entrants.
+
+Redaction: per-record values are withheld (the remainder's per-band and overall minimum and maximum amounts);
+quantiles, counts, masses and target errors are kept.