v1.9.0 — financial plan tools

Exposes okama 3.0's FinPlan — a plan modelled as an ordered sequence of portfolio stages, where each stage's terminal balance opens the next one per Monte Carlo scenario. That is what an accumulation phase followed by a withdrawal phase actually needs, and it is not expressible with the single-portfolio Monte Carlo tools.

New tools

• finplan_forecast — Monte Carlo over the whole plan: percentile bands of the wealth path, terminal-wealth statistics, survival period, probability of success against a currency threshold, per-stage boundary balances and IRR.
• finplan_backtest — the same plan replayed on historical data, returning the wealth index and the cash-flow series in nominal (fv) or discounted (pv) terms.
• plot_finplan_forecast — percentile corridor with dashed stage boundaries and stage labels, so a multi-stage chart shows where the plan changes gear.

Notes

• Stages carry their own portfolio, horizon, cash-flow strategy and return distribution, so FinPlanSpec is its own schema rather than a reuse of MCSpec.
• The chart is rebuilt through matplotlib's object-oriented API instead of calling FinPlan.plot_forecast_monte_carlo(), which goes through global pyplot and is unsafe in FastMCP's worker threads.
• No new dependencies — the okama>=3.0.0 floor is unchanged from v1.8.0.

Full Changelog: v1.8.0...v1.9.0