`Marshalling.GLOBAL_SCHEMA_FACTORY` is a mutable global singleton that leaks state across callers and tests

[shaaravraghu]

Issue type
Architecture bug

Severity
Medium

Priority
P2

Affected modules
base-marshalling

Affected files
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java

Verification status
Source inspection

Problem summary
The Marshalling interface exposes a process-wide GLOBAL_SCHEMA_FACTORY and routes all registration and default marshaller creation through it. That makes schema registration global, shared, and difficult to isolate.

Evidence
The singleton is declared at line 38.
All registration methods and newMarshaller() delegate to that singleton at lines 51-76 and 84-94.

Relevant references
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java:38
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java:51
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java:64
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java:75
base-marshalling/src/main/java/build/base/marshalling/Marshalling.java:84

Why this matters
Global mutable registration state complicates test isolation, modular embedding, and multi-tenant use inside the same JVM.
It also makes behavior depend on registration order and previous callers.

Reproduction steps

1. Review Marshalling.java.
2. Note that callers cannot create a fresh default schema factory through this API surface.
3. Observe that any registration mutates shared process-wide state.

Expected result
Library consumers should be able to create isolated schema factories and marshallers without mutating global state.

Actual result
The default API path shares one mutable schema factory across all callers in the JVM.

Suggested fix direction
Keep the global path only as a convenience layer and add a first-class instance-oriented entry point.
Document the singleton as legacy or convenience behavior if it cannot be removed immediately.

Acceptance criteria
Callers can obtain an isolated SchemaFactory and Marshaller without using global mutable state.
Tests can register schemas without impacting unrelated tests in the same process.

Suggested labels
bug
architecture
marshalling
testability