refactor(hm-exec): Model parallelism as NonZeroUsize instead of usize coerced with .max(1)

[markovejnovic]

Smell

LocalBackend::new(parallelism: usize, ..) (crates/hm-exec/src/local/backend.rs:39) stored a raw usize, and the scheduler later defended against zero with let parallelism = parallelism.max(1); before Semaphore::new(parallelism) (crates/hm-exec/src/local/scheduler.rs:136).

A tokio::sync::Semaphore::new(0) would deadlock every step, so the invariant parallelism >= 1 is real — but it was enforced only by a runtime coercion buried at the call site, and the doc comment (`0` is coerced to `1`) documented the footgun rather than removing it.

Change

• LocalBackend::new now takes parallelism: NonZeroUsize, stored as-is on the struct.
• scheduler::run takes parallelism: NonZeroUsize and constructs the semaphore as Semaphore::new(parallelism.get()) — panic-/deadlock-free by construction. The .max(1) coercion and its comment are gone.
• The CLI boundary (crates/hm/src/commands/run/mod.rs, resolve_parallelism) now produces the NonZeroUsize once: an explicit --parallelism 0 is clamped via NonZeroUsize::new(n).unwrap_or(NonZeroUsize::MIN), and the auto-detected fallback uses available_parallelism() (already NonZeroUsize). Behavior is preserved exactly: 0/absent still becomes 1/CPU-count.
• The crate's contract test was updated to pass NonZeroUsize::new(4).unwrap().

This is behavior-preserving: the same effective parallelism is selected, but zero is now unrepresentable past the CLI boundary instead of being silently corrected at the use site.

Type-safety pattern

This applies the NonZeroUsize-for-counts pattern used by fd: a count semantically required to be >= 1 should be made unrepresentable as zero at the type level rather than guarded with a runtime check at every use site. The invariant moves into the type, the semaphore construction is provably safe, and the silent coercion disappears.

CI note

Only cargo check -p hm-exec --tests was run locally (it passes). Full CI runs on this PR — in particular the hm crate's build and the TS/Python DSL build were not exercised locally.