Add fiscal-year uprating planning docs page (#1437 #1436)

changelog.d/1437.md

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+- Add a planning docs page at `docs/book/assumptions/fiscal-year-uprating-plan.md` describing the proposed move from January-1 to April-6 dates for OBR growth indices (#1437) and how it relates to the on-the-fly conversion alternative (#1436); identifies the four pipeline changes needed (yoy_growth.yaml dates, create_economic_assumption_indices.py, policyengine-core's `uprate_parameters` assumptions, and removing `convert_to_fiscal_year_parameters`) and notes the user-visible correctness gain at mid-year queries.

docs/book/assumptions/fiscal-year-uprating-plan.md

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+# Fiscal-year uprating: April-date alignment plan
+
+```{note}
+**Planning page.** PolicyEngine UK currently stores OBR growth indices
+at **January 1** dates and post-processes parameters to the fiscal year
+via `convert_to_fiscal_year_parameters`. This page captures the proposed
+move to **April 6** dates throughout, tracked under
+[#1437](https://github.com/PolicyEngine/policyengine-uk/issues/1437),
+and the on-the-fly alternative tracked under
+[#1436](https://github.com/PolicyEngine/policyengine-uk/issues/1436).
+This is a low-priority enhancement — the current approach is correct
+for annual analysis after PR #1435 extended the fiscal-year-conversion
+range out to 2040.
+```
+
+## Current pipeline
+
+The model currently does three things in sequence:
+
+1. **OBR indices** live at January 1 dates in
+   [`parameters/gov/economic_assumptions/yoy_growth.yaml`](../../../policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml)
+   (annual calendar-year growth rates) and the cumulative indices are
+   generated by
+   [`create_economic_assumption_indices.py`](../../../policyengine_uk/parameters/gov/economic_assumptions/create_economic_assumption_indices.py)
+   at January 1 dates (`f"{year}-01-01"`).
+2. **`uprate_parameters`** in `policyengine-core` writes uprated
+   parameter values at the same dates as the uprating index, so all
+   uprated parameters land at January 1.
+3. **`convert_to_fiscal_year_parameters`** in
+   [`utils/parameters.py`](../../../policyengine_uk/utils/parameters.py)
+   post-processes the parameter tree by sampling each parameter at
+   April 30 and overwriting the whole calendar year with that value.
+   This is what produces the fiscal-year-correct outputs PolicyEngine
+   ships today.
+
+This works for annual simulations but has three downsides:
+
+- **Intra-year queries are wrong.** `param("2026-03-15")` returns the
+  fiscal-year-2026 value when April 6 2026 hasn't happened yet; the
+  intended pre-April-6 value is the fiscal-year-2025 value.
+- The fiscal-year conversion is a **post-process workaround**, not a
+  correct upstream model.
+- Parameter `values_list` entries appear at January 1 dates even
+  though the legislative change usually takes effect on April 6.
+
+## Proposed change (#1437): store at April 6 throughout
+
+Update each step of the pipeline so it speaks fiscal years natively:
+
+1. Move the dates in `yoy_growth.yaml` from `YYYY-01-01` to
+   `YYYY-04-06` for every entry, ensuring the growth rate that applies
+   between fiscal year T and fiscal year T+1 sits at the start of T+1.
+2. Update `create_economic_assumption_indices.py` to emit cumulative
+   indices at April 6 dates.
+3. Confirm that `policyengine-core`'s `uprate_parameters` does not bake
+   any January-1 assumption — if it does, parametrise the date.
+4. **Remove** `convert_to_fiscal_year_parameters` and its call site in
+   [`tax_benefit_system.py`](../../../policyengine_uk/tax_benefit_system.py).
+   Once steps 1-3 land, the natural answer at every date is already the
+   fiscal-year-correct value.
+
+### What changes for users
+
+| Query | Today | After April-date move |
+|-------|-------|------------------------|
+| `param("2026")` (year-string) | Fiscal year 2026/27 value (post-process) | Fiscal year 2026/27 value (direct) |
+| `param("2026-03-15")` (pre-April 6) | Fiscal year 2025/26 value (correct) | Fiscal year 2025/26 value (correct) |
+| `param("2026-05-15")` (post-April 6) | Fiscal year 2025/26 value (**wrong**) | Fiscal year 2026/27 value (correct) |
+| `values_list` dates | January 1 | April 6 |
+
+The third row is the headline correctness gain.
+
+### Scope
+
+- Pure documentation/wiring change — **no household-impact values
+  should move** for any annual query.
+- Regression tests must include both an annual-query and an
+  April-30-style mid-year-query case so the next refactor can't
+  reintroduce the off-by-three-months bug.
+- Reform-side parameters (legislated rate changes) should already be
+  dated April 6 in their YAML files (most are); spot-check the
+  exceptions during the migration.
+
+## On-the-fly alternative (#1436)
+
+The complementary issue [#1436](https://github.com/PolicyEngine/policyengine-uk/issues/1436)
+proposes the inverse: instead of moving every date to April 6, keep
+indices at January 1 and resolve the fiscal-year conversion lazily at
+query time.
+
+That would let the conversion stop being a 26-year pre-computation step
+at startup (the `range(2015, 2041)` loop in
+`convert_to_fiscal_year_parameters`) and remove the year-range
+upper-bound altogether.
+
+The two approaches are mutually exclusive: either we change the data to
+match the queries (#1437) or we change the resolver to translate
+queries to the data (#1436). Recommendation:
+
+- Prefer **#1437 (April dates)** for correctness and ergonomics — the
+  underlying policy change happens on April 6, so the YAML should say
+  April 6.
+- Consider **#1436 (on-the-fly)** if a refactor of
+  `convert_to_fiscal_year_parameters` becomes hot-path for any specific
+  user — it's primarily a startup-cost optimisation.
+
+## Sequencing
+
+1. Land #1437 first: it removes the post-process workaround and makes
+   the YAML files self-explanatory.
+2. Once #1437 is live, evaluate whether the lazy approach in #1436 is
+   still worth doing — most of its benefit is already captured by step 4
+   above ("remove `convert_to_fiscal_year_parameters`").
+
+## References
+
+- Issue [#1437](https://github.com/PolicyEngine/policyengine-uk/issues/1437) — April-date alignment.
+- Issue [#1436](https://github.com/PolicyEngine/policyengine-uk/issues/1436) — on-the-fly conversion exploration.
+- PR [#1435](https://github.com/PolicyEngine/policyengine-uk/pull/1435) — extended the fiscal-year-conversion range to 2040.
+- [`growthfactors.md`](./growthfactors.md) — the underlying OBR growth rates.