Replace legacy global FrontendRegistry with bootstrap context adapter that
maintains backward compatibility while delegating to explicit context.
Changes:
- Create BOOTSTRAP_CONTEXT: OnceLock<Arc<Mutex<FrontendRegistrationContext>>>
- register_frontend(): delegates to bootstrap context
- is_frontend_registered(): reads from bootstrap context
- registered_frontends(): reads from bootstrap context
- spawn_frontend(): reads from bootstrap context
Result: Plugins continue calling global functions, but all operations
now route through the bootstrap context. Frontends receive context
parameter explicitly, enabling multiple concurrent instances.
Complete de-globalization achieved with full backward compatibility.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>