fix: bump gopsutil v3 → v4 to drop go-m1cpu cgo init crash on macOS 26

[mmoscosa]

Problem

On macOS 26 (Tahoe), Apple Silicon, every demux invocation crashes with SIGSEGV before main() — including demux --version and demux --help:

SIGSEGV: segmentation violation
signal arrived during cgo execution
github.com/shoenig/go-m1cpu._Cfunc_initialize()
github.com/shoenig/go-m1cpu.init.0()  go-m1cpu@v0.1.6/cpu.go:148
runtime.doInit1(...)
runtime.main()

go.mod pins gopsutil/v3, which pulls go-m1cpu (indirect). go-m1cpu reads P/E-core frequencies from IOKit in a cgo init(), and that init segfaults on macOS 26. Because it runs at package-init time, it kills the process before any code executes — no CLI flag avoids it, and CGO_ENABLED=0 is the only workaround (at the cost of losing gopsutil's libproc-backed process detection, so panes read as idle / procs: 0).

Tracked in #55.

Fix

Bump gopsutil/v3 → gopsutil/v4. v4 dropped the go-m1cpu cgo dependency in favor of sysctl, removing the crash entirely. demux imports gopsutil in only two files (internal/proc/{cwd,snapshot}.go), using only gopsutil/v3/process, and the v4 process API is source-compatible — so this is import-path + module bumps only (4 files, +15/-18).

Verification (macOS 26.5.1 / go 1.26.4 / Apple Silicon)

• Builds with CGO_ENABLED=1 (the config that crashed) and --version runs cleanly — no segfault.
• go-m1cpu is fully removed from the dependency graph (go.sum).
• go vet ./... clean; full go test ./... green.

With cgo back on, per-pane process detection works again — no more blanket idle / procs: 0.

Fixes #55.