Refactor code structure for improved readability and maintainability

docs/api/signals/reachability-contract.md

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+# Signals Reachability API Contract (draft placeholder)
+
+**Status:** TODO · awaiting Signals Guild
+
+## Scope
+- `/signals/callgraphs`, `/signals/facts`, reachability scoring overlays feeding UI/Web.
+- Deterministic fixtures for SIG-26 chain (columns/badges, call paths, timelines, overlays, coverage).
+
+## Needed from owners
+- OpenAPI/JSON schema for callgraphs and facts (request/response, pagination, ETags).
+- Reachability score model, states, and filtering parameters.
+- Fixture bundle (JSON/NDJSON) with checksums and performance budgets (target FPS/node caps).
+
+## TODO
+- Replace with ratified contract and fixtures; record schema hash/date; link from Web V and UI III logs.