build: point macOS BPF builds at a Linux VM instead of "install bpftool"

[necco-c]

What

BPF objects only build on Linux. The vendored toolchain is Linux musl-static and there is no macOS bpftool or kernel BTF, so on a Mac make bpf fell through to PATH and died at:

error: bpftool not found — install bpftool / linux-tools

That advice can't be followed on macOS. This guards the BPF/veristat rules behind uname -s: on Darwin, make bpf (and thus the default all, which yeet run invokes) fails fast with the real fix instead:

error: BPF objects build on Linux only — macOS has no bpftool or kernel BTF.
       Build inside a Linux VM, then run there. With the yeet Lima VM:
         limactl shell yeet.debian-13
         cd <project> && make && sudo yeet run -t .

clean-bpf and clangd stay on both platforms (rm/mkdir/printf need no toolchain, and clangd editor support is useful while editing .bpf.c on a Mac).

Why it matters

yeet new clones this template from the remote, so this message is what a freshly-scaffolded project shows a macOS user on their first build. Today that's the impossible "install bpftool" hint.

Verified

• macOS: make / make bpf fail fast with the VM instructions; make bundle and make clean-bpf still work.
• Linux (Lima yeet.debian-13): unchanged — the object still links (bin/probe.bpf.o, 54KB) and the bundle builds.

Unmarked commit, so this releases as a minor bump per the release convention.