Prepare v0.6.0 release

[Tinnci]

Summary

• promote the accumulated changelog entries to the dated 0.6.0 release section
• record completed P0 release gates and leave tag, GitHub Release, and PyPI verification as the final step

Validation

• merge commit CI and Security checks passed
• source distribution and wheel built successfully
• metadata and wheel contents checks passed
• isolated wheel installation and sinc-resample CLI smoke passed
• git diff --check passed

[Copilot]

Pull request overview

Prepares the repository for the v0.6.0 release by updating release-tracking documentation (roadmap status and changelog header) to reflect completed release gates and the new release section.

Changes:

• Converted the v0.6.0 “required before release” checklist in ROADMAP.md into a task-list status section with completed gates and one remaining publish step.
• Promoted the changelog’s top section from “Unreleased” to a dated 0.6.0 release heading in CHANGELOG.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
ROADMAP.md	Marks v0.6.0 release-prep gates as completed and leaves tagging/release verification as the remaining step.
CHANGELOG.md	Moves accumulated entries under a dated 0.6.0 release header.

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