⚡ Remove frame size from promise types using back-pointer deallocation

[kammce]

Problem

Previously, each promise had to store its frame size to enable manual deallocation. This required a complex handoff:

1. Context stores allocation size in variant state during operator new
2. Promise copies that size to m_frame_size member in constructor
3. Promise calls context.deallocate(m_frame_size) in self_destruct()

This handoff mechanism forced every promise type to have a non-trivial destructor and carry an extra member variable.

Solution

Store a back-pointer before each coroutine frame pointing to the context's m_stack_pointer. Now operator delete can find and update the stack pointer directly, eliminating the need for promises to track their own size.

Benefits

Removed from promises:

• m_frame_size member variable
• self_destruct() method
• Non-trivial destructor logic

Simplified context:

• State is now just blocked_by instead of variant<usize, blocked_by>
• Removes variant discriminator, saving 1 word in context size
• Direct state access (no variant checks)

Stack now uses uintptr_t: Changed from byte* to uptr* for proper alignment of back-pointers.

Added context::resume(): Convenience method for manual event loops.

Trade-off

Still allocates +1 word per frame (now for back-pointer instead of in promise), but memory layout is cleaner and promises are simpler.

Testing

Re-enabled all previously skipped tests - all passing.