Introduce a shared continuation stack for coroutine resumption.

[kovacsnador]

Summary

Introduce a shared continuation stack for coroutine resumption.

This replaces the previous parent/child continuation links with a stack head stored in SharedState. Awaited tasks now push their promise onto the shared continuation stack, and final suspension pops back to the previous continuation. TaskResumer resumes the current stack head directly instead of walking a child chain.

Changes

• Replace PromiseBase::parent / child with a conti continuation pointer.
• Add SharedState::conti as the type-erased continuation stack head.
• Update task await suspension to push awaited promises onto the shared continuation stack.
• Update final suspension to pop the completed coroutine and resume its parent continuation.
• Simplify TaskResumer by removing child-chain traversal.
• Switch SchedulablePromise from DoubleLinkable to SingleLinkable.
• Update awaiter tests for the new continuation model.
• Remove obsolete TaskResumer::FindContinuation tests.
• Add nested co_yield coverage to validate continuation behavior across awaited yielding tasks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	98.66% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
include/tinycoro/Promise.hpp	100.00% <100.00%> (ø)
include/tinycoro/PromiseBase.hpp	100.00% <100.00%> (ø)
include/tinycoro/PromiseSchedulable.hpp	95.00% <ø> (ø)
include/tinycoro/SharedState.hpp	100.00% <ø> (ø)
include/tinycoro/TaskAwaiter.hpp	100.00% <100.00%> (ø)
include/tinycoro/TaskResumer.hpp	100.00% <100.00%> (ø)
test/src/TaskAwaiter_test.cpp	100.00% <100.00%> (ø)
test/src/YieldValue_test.cpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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