Separate built-in embedding from dynamic plugin ABI exports

## Problem

Pulsar statically embeds this crate as a built-in editor. The unconditional xport_plugin! invocation emits the generic _plugin_create, _plugin_version, and _plugin_init_globals loader symbols into the rlib. Workspace test binaries that embed multiple built-in plugins then fail to link because every plugin exports the same ABI names.

## Required change

- Add an explicit uiltin Cargo feature.
- Suppress the dynamic loader ABI exports when uiltin is enabled.
- Preserve the current standalone/default cdylib behavior.
- Validate both the default plugin build and a built-in rlib check.

This is an ownership fix: static built-ins must not export process-global dynamic-loader symbols.