Fix false llvm.assumes in wake_thread! (segfault on Julia nightly)

[vchuravy]

Fixes #66.

#64 changed the bound in wake_thread! to assume(unsigned(length(Base.Workqueues())) > unsigned(tid)). On Julia 1.13+, Base.Workqueues() is the current thread's work queue, and its length is the number of tasks queued on that thread, not the number of threads. So the assumption is false, which is undefined behaviour.

• Nightly (LLVM 22): LLVM acts on the assumption. It deletes the empty-list check in the inlined length(::IntrusiveLinkedList) loop, which then segfaults (Fault at memory address: 0x1d3) on the first Polyester.@batch launch with 2 or more threads.
• Julia 1.13 (LLVM 20): has the same undefined behaviour but happens not to crash.

This PR:

• Assumes Threads.maxthreadid() > tid instead. The function reads Base.Workqueues[tid + 1], which needs exactly that bound.
• Fixes the off-by-one in assume(length(TASKS) > tid + 1). That assumption is false for the last worker (always at -t 2). It is now length(TASKS) >= tid, which matches the TASKS[tid] access.
• Adds comments explaining both bounds.
• Runs the existing Julia nightly workflow (ci-julia-nightly.yml) weekly on a schedule, and allows starting it by hand.

Testing

JULIA_NUM_THREADS=4, Pkg.test():

Julia	before	after
1.10.12	pass	pass
1.12.7	pass	pass
1.13.1	pass	pass
1.14.0-DEV.3384	segfault in wake_thread!	pass

With the fix, the reproducers in #66 and Trixi.jl (Polyester backend, -t 2 and -t 8) also run on nightly.

The nightly workflow (ci-julia-nightly.yml) runs only on pull requests and pushes to main. Its last run was on 2026-05-29, before the nightly LLVM update that exposes this bug, so it never saw the crash. The second commit adds a weekly schedule (and workflow_dispatch) to that workflow so that changes in Julia nightly are caught without a change in this repository. A patch release after merging would unblock Polyester users on nightly.

This PR was prepared with assistance from Claude Code (Anthropic's AI coding assistant). I reviewed the changes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (cb9cf25) to head (4f1a898).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   99.20%   94.44%   -4.77%     
==========================================
  Files           4        4              
  Lines         126      126              
==========================================
- Hits          125      119       -6     
- Misses          1        7       +6     

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[ranocha]

Thanks a lot!