Makefile: declare every target phony, and stop hiding integ-arm64 from help

[calvinchengx]

Two small Makefile fixes, independent of each other and of #37.

.PHONY listed only integ

Every other target was a real file target. A file or directory named test, run, fmt, lint or help in the repository root silently turns the matching target into a no-op — and it exits 0, so CI would go green without running anything:

$ make test
tests ran
$ touch test
$ make test
make: `test' is up to date.
$ echo $?
0

test is a plausible name for someone to create. Now all seven targets are declared.

make help hid integ-arm64

The pattern was ^[a-zA-Z_-]+: — no digit class — so integ-arm64 carried a ## comment that make help never printed. Adding 0-9 fixes it, and it stays correct for any future target with a version or architecture in its name.

One comment, no behaviour change

I also added a header saying what the Makefile is: convenience assuming GNU make and a POSIX shell, rather than the build. The build and tests are the Go toolchain alone, which is what lets the Windows CI job run go test ./... directly. That felt worth stating where someone on Windows would look first, but it's a comment — drop that commit if you'd rather not carry it.

Verified with make help and make test.

[calvinchengx]

Already on main, in fuller form: the header comment, the complete .PHONY list and the a-zA-Z0-9_- fix in help (line 52) are all there. Closing as landed, not as rejected.