fix(ci): stop the Dart suite racing the runtime's global panic hook

[Peefy]

Summary

• Run the Dart test suite serially (dart/dart_test.yaml, concurrency: 1) — dart test runs test files in parallel isolates that share one libkcl, and overlapping evaluations clobber each other's panic hook, so a KCL error can come back with an empty message
• Document the constraint in dart/README.md next to the existing not-thread-safe note
• Drop the duplicated windows-latest matrix entry, which ran the Windows leg twice

The failure

Run #39233061134, Test on macos-15-intel:

❌ test/kcl_lib_test.dart: plugins reports an unknown method as a KCL error rather than crashing
Expected: contains 'nope'
  Actual: 'EvaluationError\n---> File ./__main__.k:2: \n\n'

thread '<unnamed>' panicked at crates/runtime/src/stdlib/plugin.rs:80:13:
invalid method: kcl_plugin.strings.nope is not found

Why

A plugin method that was never registered is reported the way every other KCL failure is — as a __kcl_PanicInfo__ object the plugin agent returns, which the runtime turns into a Rust panic. The panic's payload is what errMessage is built from, and the runtime captures it with a process-global std::panic hook that it takes, replaces and restores around every evaluation, recording into a thread-local.

Two overlapping evaluations therefore race on that hook: one thread restores the hook another is running under, and the panicking thread reads back an empty record, so the error renders with no message. Only the first panic on a thread exposes it — which is why the very next plugin test (reports a throwing method as a KCL error) passed.

Reproduced locally against libkcl v0.13.2 with four isolates (one driving plugin errors with a distinct method name per round, three hammering ordinary evaluations): 2 of 3 runs hit the race, one of them byte-identical to the CI failure at round 0.

Serial execution costs ~0.1s on this package.

Note

This stops the flake but is not the real fix: the hook install/restore is not concurrency-safe in kcl-lang/kcl (crates/runner/src/runner.rs), and FastRunner::run falls back to unwrap_or_default() instead of the caught panic payload when the record comes back empty. That belongs upstream and needs a new libkcl to take effect here.

Test plan

• make test in dart/ — 141 tests pass
• --reporter=expanded output shows each file's results as one contiguous block, i.e. suites really are serial now
• matrix parses to 5 legs; windows-latest appears once

🤖 Generated with Claude Code