internal/checker: SOCKS5 primitives + tests

socks5Greeting/Auth/Connect/UDPAssociate per docs/superpowers/specs/
2026-05-01-checker-design.md. RFC 1928 + RFC 1929 wire bytes, raw
reply bytes returned on every error path for RawHex display, ctx
deadline applied via SetDeadline, ctx.Err() joined into error chain
on cancellation. Sentinel errors and ErrSocks5Reply{Code} for code
matching via errors.Is.

Tests: 22 subtests with fake net.Listen server, table-driven per
primitive (happy paths, REP codes, short reads, bad version,
oversize input rejection without I/O, ctx-cancel mid-read).
go test -race -cover passes at 89.0%, go vet clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.mod

@@ -5,6 +5,7 @@ go 1.23
 require (
 	github.com/minio/selfupdate v0.6.0
 	github.com/spf13/cobra v1.10.2
+	github.com/stretchr/testify v1.11.1
 	github.com/wailsapp/wails/v2 v2.12.0
 	golang.org/x/mod v0.23.0
 	golang.org/x/sys v0.30.0
@@ -14,6 +15,7 @@ require (
 	aead.dev/minisign v0.2.0 // indirect
 	git.sr.ht/~jackmordaunt/go-toast/v2 v2.0.3 // indirect
 	github.com/bep/debounce v1.2.1 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/go-ole/go-ole v1.3.0 // indirect
 	github.com/godbus/dbus/v5 v5.1.0 // indirect
 	github.com/google/uuid v1.6.0 // indirect
@@ -30,6 +32,7 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/pkg/browser v0.0.0-20240102092130-5ac0b6a4141c // indirect
 	github.com/pkg/errors v0.9.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/samber/lo v1.49.1 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
@@ -41,4 +44,5 @@ require (
 	golang.org/x/crypto v0.33.0 // indirect
 	golang.org/x/net v0.35.0 // indirect
 	golang.org/x/text v0.22.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )