Remove coroutine support

[chklauser]

Summary

• remove the coroutine and yield syntax, AST lowering, newcor opcode, and coroutine VM/runtime contexts
• replace coroutine-backed native sequence commands with ordinary lazy PValueEnumerable values
• add the JIT-compatible create_enumerable(getEnumerator) command and migrate repository scripts to it or existing sequence combinators
• make coroutine and yield available as ordinary identifiers

Compatibility

This intentionally removes the coroutine language/runtime feature. Script code that needs custom lazy enumeration can return:

create_enumerable(() =>
    new enumerator(moveNext, current, dispose)
)

Validation

• dotnet test --no-restore
• 1,435 total: 1,432 passed, 3 skipped, 0 failed
• stale implementation-symbol scan and git diff --check