Create parent dirs for Ninja file

[leynos]

Summary

• ensure parent directories exist before writing a Ninja manifest
• test emitting a Ninja file into a non-existent directory

Testing

• make fmt (fails: AGENTS.md:153 MD040/fenced-code-language Fenced code blocks should have a language specified)
• cargo fmt --all
• make lint
• make test

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https://chatgpt.com/codex/tasks/task_e_689671720f9483228ad0e3782d5390a5

Summary by Sourcery

Create missing parent directories prior to emitting Ninja manifests and verify the behavior with a new unit test.

New Features:

• Ensure parent directories are created before writing a Ninja file

Tests:

• Add test to verify Ninja emission into a nested non-existent directory