ci: path-filtered builds and changelog-driven release body

[thorinf]

Summary

Two CI improvements bundled into one PR.

Path-filtered builds

Split build.yml into per-component workflows so each only runs when its files change:

• build-firmware.yml — triggers on firmware/**, protocol/**, or its own file
• build-vst.yml — triggers on vst/**, protocol/**, or its own file
• Added make -C vst/tests test to the VST build (wasn't running before)

Effect: docs-only PRs skip both builds, firmware-only PRs skip the slow macOS VST job, VST-only PRs skip the Ubuntu firmware job. Protocol changes still trigger both.

Changelog-driven release body

Release workflow now extracts the matching version's section from CHANGELOG.md and uses it as the GitHub release body, instead of auto-generating notes from commit messages. Fails the release if CHANGELOG.md has no entry for the version being tagged.

Notes

• If branch protection has a required check named "Build", update it to require the new job names.
• Re-tag v1.0.0 after this lands to actually cut the release with the proper changelog content.