Prepare mere.run 0.38.0 release

[a-github-name]

Summary

• bump the CLI, app fallback, and release contract test to 0.38.0
• move the completed Unreleased notes into the 0.38.0 changelog entry
• record the exact merged PR range (Redesign the documentation experience #293-Add native Qwen3.8 27B model lanes #303)

Apparently the stars aligned: mere.run 0.38.0 is also the Qwen3.8 release. Versioning has achieved product-market fit. ✨

Validation

• ./scripts/check.sh
• 2,862 tests executed, 207 expected asset/GPU skips, 0 failures
• strict SwiftLint, build, CLI help sweep, and hygiene scans passed

Release boundary

This PR prepares the source version and changelog only. The annotated v0.38.0 tag will be created from the protected merge commit after exact-head CI and merge-tree verification.