Task: Give the mixture a simple mode

The Qt FX rate editor offers a simple mode next to the raw component table: sliders for the global trend drift, the global volatility and the jump event frequency, which build the Gaussian mixture, and for the mean reverting engine a κ slider on a log scale with the half-life read out and a σ slider. A quant who does not think in μ, σ and w gets a feed that behaves as they describe. The web mixture tab has only the raw table.

• The mixture tab offers Simple and Advanced, and the simple controls write the same components the table holds.
• A mixture the simple controls cannot express is shown in Advanced only, with a note that says so.
• The mean reverting engine gets its own simple controls: κ by half-life and σ.
• The preview and the save work the same from either mode.
• The switch is the shared EditingModeToggle and useEditingMode (ui/EditingMode.tsx), which the IR process tab already uses. A person who picks Advanced there gets Advanced here too, and the other way round.
• The conversion between the sliders and the components is the service's, like convert_fx_mixture. The screen does no model arithmetic.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

The FX price behaviour tab offers Simple and Advanced. Simple asks how far the price drifts, how far it moves and how often it jumps in an hour, or, for the mean-reverting engine, how long it takes to come half way back. The service converts both ways (convert_fx_simple). The switch and its saved choice are shared with the IR Process tab. A mixture that only approximates the simple form is converted when the person switches to Simple, and a toast says what changed. PR 3152.