Simplify thunks and remove continuations

[chklauser]

Summary

• remove the unused Continuation runtime type
• replace the cooperative thunk trampoline with Lazy<PValue> using ExecutionAndPublication
• preserve thunk, asthunk, and force, including nested-thunk forcing
• cache both successful results and exceptions with .NET Lazy semantics
• remove CooperativeContext and its dedicated FunctionContext escape hatch
• update the runtime overview and add memoization regression tests

Validation

• dotnet test --no-restore --tl:off --filter 'FullyQualifiedName~ThunkTests' (24 passed)
• dotnet test --no-restore --tl:off (1,441 total; 1,438 passed; 3 skipped)
• dotnet csharpier check .