feat(BT-002): add historical snapshot provider

docs/EXEC-001A_BT-001_MVP_ARCHITECTURE.md

@@ -700,7 +700,8 @@ If this changes later, it must be a scenario-level parameter.
 ## 5. Continuous-vs-listed quantity must be explicit
 
 MVP synthetic runs may use `continuous_units`.
-BT-002 listed snapshot runs should support `listed_contracts` with contract-size rounding.
+The shipped BT-002 provider slice also remains `continuous_units`-only.
+`listed_contracts` with contract-size rounding is deferred to follow-up slice `BT-002A`.
 
 Do not hide rounding rules inside providers.
 They belong in the position sizing logic and must be recorded in the result.
@@ -714,9 +715,9 @@ Do not collapse the entire hedge economics into end-of-period payoff only.
 
 Any missing snapshot/mark fallback must add:
 
-- a run warning
+- a run or template warning
 - a template validation note
-- a deterministic result status if the template becomes incomplete
+- and, in a fuller follow-up slice, a deterministic result status if the template becomes incomplete
 
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