Add canonical Json format and external format binding

proposals/serialization_format_registry.md

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+# Serialization Format Registry
+
+Current status:
+
+- no global serialization-format registry
+- formats are exported explicitly from modules and used explicitly, e.g. `Lynon.encode(...)`, `Json.decode(...)`, or `MyFormat.encode(...)`
+
+Why no registry now:
+
+- explicit module exports already solve the current use case
+- a registry adds global mutable state and naming semantics we do not currently need
+- there is no current runtime feature that needs format discovery by string name
+
+When a registry may become worth adding:
+
+- config-driven format selection, e.g. `"format": "json"`
+- host-side introspection such as "list installed serialization formats"
+- collision detection across independently loaded modules
+- admin or tooling APIs that need to resolve a format without importing its module explicitly
+
+If added later, the registry should be:
+
+- optional, not required for normal explicit usage
+- based on stable fully qualified ids, not just short export names
+- designed as a host/tooling facility first, not as part of ordinary script-level serialization