Task: Derive the risk reversal and strangle ladders

A party runs a risk reversal ladder and a strangle ladder, each derived from one stored surface run and reproducible from it alone.

• The derivation states the change of basis it applies, cites the smile identity on FX Volatility Surface, and names the surface points it reads.
• The strangle ladder reads the butterfly style the pair is stored with, and refuses a pair whose style it cannot convert rather than assuming one.
• The derived figure, its inputs and the run it came from are all stored, so the figure can be recomputed without re-running the engine.
• A test proves the derived risk reversal equals the difference of the two surface-point deltas on a worked example.

(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.

• Greek ladder reports — the design this task builds.
• FX Volatility Surface — the ATM, risk reversal and butterfly identity the derivation applies.