docs: migrate roadmap to structured yaml tasks

docs/roadmap/backlog/CORE-001D-decimal-boundary-cleanup.yaml

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+id: CORE-001D
+title: External Boundary and Persistence Cleanup for Decimal Unit Types
+status: backlog
+priority: P2
+effort: M
+depends_on:
+  - CORE-001B
+  - CORE-001C
+tags: [core, decimal, persistence]
+summary: Make Decimal/unit-safe values reliable across persistence, APIs, and provider boundaries.
+acceptance_criteria:
+  - Persistence format for unit-safe values is explicit and stable.
+  - Decimal-bearing JSON/API serialization is documented and tested.
+  - Float-heavy integrations have named conversion boundaries.
+  - Remaining raw-float domain hotspots are identified or removed.