#66: Add ZEND_ASYNC_SCHEDULER_LAUNCH() to TrueAsync public API

[EdmondDantes]

   Expose async_scheduler_launch() function through the TrueAsync API to allow
   extensions to explicitly initialize and start the scheduler before creating
   coroutines. This is particularly useful for extensions that need deterministic
   scheduler initialization timing.

   Changes:
   - Add zend_async_scheduler_launch_t function pointer type
   - Add ZEND_ASYNC_SCHEDULER_LAUNCH() macro for scheduler initialization
   - Register scheduler_launch_fn as first parameter in zend_async_scheduler_register()
   - Update CHANGELOG.md with API addition in version 0.5.0

[Copilot]

Pull Request Overview

This PR adds ZEND_ASYNC_SCHEDULER_LAUNCH() to the TrueAsync public API to allow extensions to explicitly initialize and start the scheduler before creating coroutines, providing deterministic scheduler initialization timing.

• Added function pointer type and macro for scheduler initialization in the public API
• Updated scheduler registration to include the launch function as a parameter
• Added performance optimization using UNEXPECTED() macro for fiber check

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
scheduler.c	Performance optimization with UNEXPECTED() macro for fiber check
async_API.c	Updated scheduler registration to include async_scheduler_launch function
CHANGELOG.md	Documented the new API addition in version 0.5.0

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