make: test and check targets

[vynulldev]

What & why

Two new Makefile targets. make test runs the unit suite. make check is the full pre-release gate: build, vet, a gofmt guard, the unit tests, and then the e2e suite - one command where the release ritual previously ran five.

The gofmt step is a real guard, not just a listing: plain gofmt -l prints offending files but exits zero, so it passes silently in a command chain. The target fails the run and names the files. Verified by mutation: a deliberately misformatted file makes make check exit non-zero and print the path; removing it goes back to green.

make check was run end to end on this branch, including the e2e suite (which still needs ffmpeg and the DJ-Link UDP ports free, as documented on the e2e target).

Hardware testing

• Tested on: N/A: Makefile only, no code change.

Checklist

• go build ./..., go vet ./..., and go test ./... pass
• gofmt -l . is clean
• New source files carry an SPDX header (GPL-3.0-or-later) (n/a, no new files)
• Tested on real hardware (deck + firmware noted above), or this change doesn't affect deck behaviour
• I agree my contribution is licensed under the project's GPLv3