## Why
The schema exporter sits on application builds' critical path, but compiling it as a test harness also compiles and registers unit tests that schema generation does not need.
## What changed
Build the exporter with `--cfg=test` to retain test-only schema derives without the test harness, and disable its debug metadata. Run schema generation directly on a thread with an 8 MiB stack.
Share the environment-driven schema writer with the existing ignored test, allow a configurable crate root in `schema_sources`, and switch the public schema bundle to the dedicated generator.
GitOrigin-RevId: 317e24f77e153b9ba0c5c583837eed3bcd3774fd
