why

The compiled runtime-path test uses POSIX fixture paths, and resolveRuntimeTasksRoot() intentionally returns a forward-slash path for compiled callers. Building the expected value with host-native path.join() made that assertion fail deterministically on Windows.

Fixes #2734

what changed

• Build the compiled fixture expectation with path.posix.join().
• Preserve the existing host-native expectation for the source-mode branch.
• Leave runtime behavior unchanged.

test plan

• vitest run packages/evals/tests/runtimePaths.test.ts --reporter=verbose (2 passed on Windows)
• Evals TypeScript type-check
• Targeted Oxfmt and Oxlint checks
• git diff --check
• Full evals suite attempted: 414 passed; 6 unrelated tests timed out under the aggregate Windows run.

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Summary by cubic

Makes the compiled runtime-path test portable by building the expected compiled path with path.posix.join() to match forward-slash outputs. Previously the test used host-native path.join(), which produced backslashes on Windows and failed deterministically; the source-mode branch still uses host-native joining and runtime behavior is unchanged.