Version flag, Makefile, and honest cross-platform documentation

Neither binary had a -version flag - a basic expectation for any CLI
tool, and useful for anyone reporting a bug ("which build is this").
internal/version holds Version/Commit/Date, set via -ldflags "-X
mitmux/internal/version.X=..." at build time and defaulting to "dev"
for a plain `go build` with no ldflags, so -version is never blank or
misleading about whether a given binary is a tagged release or a local
build. Both mitmux and mitmuxd gained a -version flag that prints it
and exits.

Makefile: `make build` (both binaries for the current platform, version
info from `git describe`), `make test` (the same build/vet/gofmt/test
checks expected before every commit here), `make install` (a thin
wrapper over `go install`, respecting GOBIN/GOPATH as usual - not
reimplementing Go's own path resolution), `make release` (cross-compiles
both binaries for linux/darwin/windows/freebsd, amd64+arm64 where it
makes sense, into dist/). Every target is CGO_ENABLED=0:
modernc.org/sqlite is pure Go, so no C toolchain is needed anywhere,
cross-compiling included - this was already true before this commit,
just not verified or made easy to use.

Verified live, every target actually run rather than just written:
`make build` produces working binaries with version info correctly
picked up from git (confirmed against a real -version invocation, both
the "dev" default and an ldflags-injected release-style version
string); `make test` runs clean; `make release` was run for real and
produced 6 platform/arch binaries, each confirmed with `file` to be a
genuinely correctly-formatted executable for its target (Mach-O for
both macOS architectures, PE32+ for Windows, ELF for both Linux
architectures and FreeBSD) - not just "the command exited zero." `make
install`'s correctness rests on `go install` itself, Go's own
well-tested mechanism; deliberately not run for real here since it
writes into the real GOPATH/bin outside this repo, unprompted.

README gained an honest Platforms section: Linux is what's actually
been run and verified throughout this project's development; macOS,
Windows, and FreeBSD cross-compile cleanly and pass go vet, and the
code has nothing Linux-specific in it (CA/history storage already used
Go's own cross-platform os.UserConfigDir, not a hardcoded XDG path -
also fixed the README's install-directory example, which had been
Linux-only text), but they haven't run on real hardware, so they're
documented as "should work, not yet verified" rather than a claim this
session can't actually back up. Also flagged a concrete, real gotcha:
macOS's shorter Unix domain socket path limit combined with the deeper
~/Library/Application Support default control-socket location could
matter for a long username, with the existing -socket flag as the
workaround.

go build/vet/gofmt/test/mod tidy all clean.

1 files changed, 32 insertions, 0 deletions

diff --git a/internal/version/version_test.go b/internal/version/version_test.go
new file mode 100644
index 0000000..d2d501b
--- /dev/null
+++ b/internal/version/version_test.go
@@ -0,0 +1,32 @@
+package version
+
+import "testing"
+
+func TestStringDefault(t *testing.T) {
+	orig := Version
+	Version, Commit, Date = "dev", "", ""
+	defer func() { Version = orig }()
+	if got := String("mitmux"); got != "mitmux dev" {
+		t.Errorf("String(%q) = %q, want %q", "mitmux", got, "mitmux dev")
+	}
+}
+
+func TestStringWithCommitAndDate(t *testing.T) {
+	origV, origC, origD := Version, Commit, Date
+	Version, Commit, Date = "1.2.3", "abc1234", "2026-01-02"
+	defer func() { Version, Commit, Date = origV, origC, origD }()
+	want := "mitmuxd 1.2.3 (abc1234, 2026-01-02)"
+	if got := String("mitmuxd"); got != want {
+		t.Errorf("String(%q) = %q, want %q", "mitmuxd", got, want)
+	}
+}
+
+func TestStringCommitOnly(t *testing.T) {
+	origV, origC, origD := Version, Commit, Date
+	Version, Commit, Date = "1.2.3", "abc1234", ""
+	defer func() { Version, Commit, Date = origV, origC, origD }()
+	want := "mitmux 1.2.3 (abc1234)"
+	if got := String("mitmux"); got != want {
+		t.Errorf("String(%q) = %q, want %q", "mitmux", got, want)
+	}
+}