build: support Windows and CGO_ENABLED=0 targets

[psycep]

Summary

Finishes what #5 started. Makes go build ./... succeed on Windows (GOOS=windows CGO_ENABLED=0) and on Linux with CGO_ENABLED=0, without forcing users through WSL. Pairs with the -proxy flag from #10 so Windows and cgo-off users have a proxy path at all.

Builds on the direct_portable.go scaffolding already landed in #10, which provides the pure-Go dialer non-cgo / Windows builds fall back to. This PR just tags the three tools that can't follow that path, gives them stubs, teaches the installer about the new targets, and updates the README accordingly.

Changes

• Build tags on the three affected tools:

  • tools/sniff/main.go, tools/split/main.go: tagged !windows && cgo (they import github.com/google/gopacket/pcap, which needs libpcap via cgo). New main_stub.go tagged windows || !cgo.
  • tools/sniffer/main.go: tagged !windows (Unix raw sockets via syscall.Socket + IP_HDRINCL, no cgo dependency). New main_windows.go tagged windows.
  • Each stub prints a one-line "not supported on this build" message and exits 1, so go build ./... always produces a binary and the install layout stays consistent across platforms.

• install.sh gets a --target flag:

  • --target native (default): today's flow, Linux/macOS cgo, install to /usr/local/bin.
  • --target portable: CGO_ENABLED=0 for the host OS, output in ./dist/portable/.
  • --target windows: GOOS=windows CGO_ENABLED=0, .exe output in ./dist/windows/.
  • --target all: build every target in one run.
  • When run with no flags and a TTY on stdin, the installer prints an interactive menu explaining each target briefly (tool set, proxy path, output location). Non-TTY invocations (CI, pipes) default silently to native so existing automation keeps working.

• README updates:

  • Installation section gains example --target invocations.
  • New "Platform Support" table shows the three build configurations, which tools are available in each, and which proxy path works.
  • Dropped a stale #9 section-number reference in the Proxy Support section.

Not changed

• install.sh's libpcap check stays for the native target. Cross-compile targets skip it (the stubs handle the absence). The existing sniff/split skip-when-libpcap-missing behavior is preserved for native builds.
• pkg/third_party/smb2 handles its own platform concerns; no tags needed from us.
• The cross-compile binaries use raw tool names (ping.exe, net.exe, etc). These collide with Windows built-ins when run from the same directory in cmd.exe. The native install already prefixes with gopacket-; doing the same for cross-compile outputs is queued as a follow-up.

Test plan

• go build ./... clean (default, cgo=1 Linux)
• CGO_ENABLED=0 go build ./... clean
• GOOS=windows CGO_ENABLED=0 go build ./... clean
• go vet ./... clean across all three configurations
• go test ./pkg/transport/ passes, 13/13
• CGO_ENABLED=0 go test ./pkg/transport/ passes, 13/13, exercising the portable directDial path end-to-end
• Live Windows smoke test against a small single-DC AD lab. Cross-compiled .exe binaries confirmed working: DumpNTLMInfo (SMB2 negotiate + NTLM info), lookupsid (SAMR over SMB named pipe), samrdump (SAMR user/group enum), rpcdump (RPC epmapper, 435 endpoints listed). Four different protocol stacks exercised via the pure-Go net.Dialer path on Windows.