♻️ Decouple context from virtual callback API via unblock_listener

[kammce]

Separate the unblock notification mechanism from the context class into a dedicated unblock_listener interface. This eliminates the need for virtual function on context itself, allowing each context to be a simple, concrete object.

The callback pattern is now only used for the unblock event—the only notification that must happen asynchronously (in ISRs or other threads). The scheduler can directly inspect context state to determine readiness and sleep duration without callbacks.

Changes:

• Extract unblock notification into unblock_listener interface
• Remove virtual methods from context class
• Update sleep_duration to use microseconds (u32) instead of nanoseconds to save 4 bytes, reduce the timing requirements on the scheduler, and to provide a more realistic delay range for most systems.
• Simplify context state inspection for scheduler decision-making
• Add on_unblock() listener registration/clearing on context
• Update tests to use new listener-based design