Simulation engine: axes sweeps + dynamics hook

[vahid-ahmadi]

Addresses #49.

Scope

Two new, fully-tested engine capabilities, built on top of the existing Simulation::branch + Comparison primitive (reused, not duplicated):

Axes (parameter / input sweeps) — src/engine/axes.rs

Mirrors PolicyEngine's "axes" concept. An Axis is (name, min, max, count, setter):

• Axis::on_people(...) / Axis::on_people_where(..., select, ...) — sweep an input field (e.g. employment income £0→£200k) over all / selected people.
• Axis::on_parameters(...) — sweep a parameter value (e.g. the basic income-tax rate).

Simulation::run_axis(&axis) linearly spaces the range, applies the setter per step, runs the engine in parallel with rayon, and returns one AxisStep { index, value, results } per point — the per-step results that back marginal-tax-rate, cliff-edge, and budget-constraint charts.

Dynamics hook — src/engine/dynamics.rs

A clean Dynamics trait for a behavioural-response pass applied between the baseline run and the reform run; apply_dynamics(passes, ...) threads passes in order over the input frame (empty = static run).

• LabourSupplyDynamics — formalises the existing OBR Slutsky-decomposition apply_labour_supply_responses behind the trait, so the labour-supply formulas finally have a hook. No logic duplicated; honours labour_supply.enabled.
• TakeUpDynamics { target_rate, benefits } — a deterministic benefit take-up campaign that flips would_claim_* flags for benefit units whose migration_seed is below target_rate (consistent with the engine's existing deterministic take-up gate).

Additive only — existing engine behaviour and main.rs are unchanged.

Tests

10 new unit tests (cargo test → 201 passed, 0 failed across both test binaries):

• Earnings sweep yields monotone non-decreasing net income and monotone income tax.
• Parameter sweep: raising the basic rate raises tax.
• on_people_where only touches the selected household.
• value_at range / single-step edge case.
• Dynamics: empty pass is identity; labour-supply pass raises earnings on an NI cut and is a no-op when disabled; take-up campaign raises benefit spending and is a no-op at rate 0.

Remaining

• Branched simulations — already existed (Simulation::branch + Comparison); reused here.
• Axes — done.
• Labour-supply dynamics — formula already existed; now formalised behind the Dynamics trait.
• Take-up dynamics — minimal deterministic implementation done; stochastic take-up and a full apply_dynamics ordering wired into main.rs (income → MTR → response → recompute) remain follow-up slices.

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