Turns out I missunderstood standby mode. It does not actually reset the device. It can reset on enter but that is not what I want. SRAM1 will be clear so What I want is some kind of "soft reset".
Looking at embassy-boot-stm32 I see something like
```rs
        let mut p = cortex_m::Peripherals::steal();
        #[cfg(not(armv6m))]
        p.SCB.invalidate_icache();
        p.SCB.vtor.write(start);

        cortex_m::asm::bootload(start as *const u32)
```

Could I use this to "run main again"?