`CycleStrategy::FallbackImmediate`: Investigate shuttle failures

[MichaReiser]

Here's the failing tests:

test panicked in task 'main-thread'
failing schedule:
"
91028003c1bbbd9085d996cdd101000000004948a22449a22492922849924449122949122589
244952a22449[1249](https://github.com/salsa-rs/salsa/actions/runs/15212059093/job/42788411582?pr=876#step:4:1250)49942449922491242589922449a224499424491245922449929224499224
4992244992244992244992244992244992244992244992249122494a12498924499224495212
4949224992244592244952924489924892244949222581244992244992244992244992244992
244992244912
"
pass that string to `shuttle::replay` to replay the failure

thread 'cycle_a_t1_b_t2_fallback::the_test' panicked at tests/parallel/cycle_a_t1_b_t2_fallback.rs:64:9:
assertion `left == right` failed
  left: (1, 9)
 right: (1, 2)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2025-05-23T14:00:12.752542Z ERROR execution{i=241}:step{task=0}: generator::detail::gen: set panic inside generator    
2025-05-23T14:00:12.752688Z  INFO shuttle::scheduler::metrics: run finished iterations=242 steps=[min=295, max=384, avg=314.1] context_switches=[min=4, max=62, avg=48.7] preemptions=[min=0, max=54, avg=41.6] random_choices=[min=0, max=0, avg=0.0]

I dunno what you're planning on doing, but maybe we should just land this branch and fix failing tests as we build out the Shuttle test suite?

Originally posted by @davidbarsky in #876 (comment)

[MichaReiser]

CC: @ChayimFriedman2

[MichaReiser]

Oh, it looks like #882 fixes those issues too. I don't feel very confident about my changes related to fallback immediate (I don't really understand it and the 100 FIXME comments aren't very helpful because they don't explain why things don't apply to fallback immediate.