docs: draft release notes

[marcus]

Summary:

• Add an Unreleased changelog section for post-v0.3.4 changes.
• Preserve existing changelog style and PR references.

Validation:

• git diff --check
• commit pre-commit hook: go vet, go build

Notes:

• Docs-only CHANGELOG.md update; Go tests were skipped per plan because no non-doc files were touched.

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