Refactor code structure for improved readability and maintainability

docs/api/graph/overlay-schema.md

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+# Graph Overlay & Cache Schema (draft placeholder)
+
+**Status:** TODO · awaiting Graph Platform Guild ratification
+
+## Scope
+- Overlay/cache schema for graph tiles used by Web gateway and UI overlays.
+- Validation rules for bbox/zoom/path; pagination tokens; deterministic ordering.
+- Error codes and sampling/telemetry fields.
+
+## Needed from owners
+- JSON schema (or OpenAPI fragment) for overlay response and cache metadata.
+- Allowed zoom levels/coordinate system; max nodes/edges per tile; hashing/etag rules.
+- Sample overlay bundle (happy path + rate-limit + validation error).
+
+## TODO
+- Insert ratified schema + samples; note schema hash/date; link from Web II sprint log.