Keep size-one parallel loops in the task system

[abadams]

Summary

A loop marked .parallel() is expected to be dispatched through the task system even when its extent turns out to be one. Among other things, this makes .parallel(Var::outermost()) a useful idiom for isolating a body into its own task/function without otherwise changing the loop nest.

Two passes were collapsing such loops into a serial body:

• Simplify turned a For whose bounds prove it runs at most once into a LetStmt. It now leaves parallel loops alone.
• RemoveLoopsOverOutermost stripped the always-size-one .__outermost loop unconditionally. It now leaves parallel loops alone too — otherwise the .parallel(Var::outermost()) idiom never reaches the task system even after the simplifier change.

Test plan

• New correctness/parallel_size_one test installs a custom halide_do_task and asserts it fires both for a statically-size-one parallel loop (f.compute_at(g, x).parallel(x)) and for a parallel outermost loop (f.parallel(Var::outermost())).
• Verified the test fails on the pre-change library (task handler never called) and passes after the change.

🤖 Generated with Claude Code

[alexreinking]

If this causes performance regressions anywhere, somehow, I'd say, "if you ask for a parallel loop, don't be surprised when it when it is one."

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.98%. Comparing base (5f95e0c) to head (2885977).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9454      +/-   ##
==========================================
- Coverage   70.12%   69.98%   -0.14%     
==========================================
  Files         261      261              
  Lines       79938    79941       +3     
  Branches    19478    19481       +3     
==========================================
- Hits        56053    55944     -109     
- Misses      18056    18086      +30     
- Partials     5829     5911      +82     

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