Add the pack package: mirror to zim or self-contained binary

BuildZIM walks a cloned host directory, turns every file into a content
entry with a MIME inferred from its extension, picks a main page, and adds
the standard metadata plus a mainPage redirect. state.json is skipped.
BuildBinary appends the archive to a copy of kage behind a KAGEPCK1 trailer,
and Embedded detects that trailer at startup so the binary serves itself.
Handler maps / to the main page and /path to a content entry, the same
handler the embedded viewer uses.

pack/embed.go

@@ -0,0 +1,50 @@
+package pack
+
+import (
+	"encoding/binary"
+	"io"
+	"os"
+)
+
+// Embedded inspects the running executable for an appended ZIM archive. If the
+// KAGEPCK1 trailer is present, it returns a ReaderAt bounded to the archive, its
+// size, and ok=true; the file handle stays open for the life of the process so
+// the viewer can serve from it. A normal kage build has no trailer, so the cost
+// to every ordinary invocation is one Open plus a 24-byte ReadAt.
+func Embedded() (ra io.ReaderAt, size int64, ok bool) {
+	exe, err := os.Executable()
+	if err != nil {
+		return nil, 0, false
+	}
+	f, err := os.Open(exe)
+	if err != nil {
+		return nil, 0, false
+	}
+	info, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+		return nil, 0, false
+	}
+	total := info.Size()
+	if total < int64(trailerLen) {
+		_ = f.Close()
+		return nil, 0, false
+	}
+
+	tr := make([]byte, trailerLen)
+	if _, err := f.ReadAt(tr, total-int64(trailerLen)); err != nil {
+		_ = f.Close()
+		return nil, 0, false
+	}
+	if string(tr[:8]) != trailerMagic || string(tr[trailerLen-8:]) != trailerMagic {
+		_ = f.Close()
+		return nil, 0, false
+	}
+	zlen := int64(binary.LittleEndian.Uint64(tr[8:16]))
+	start := total - int64(trailerLen) - zlen
+	if zlen <= 0 || start < 0 {
+		_ = f.Close()
+		return nil, 0, false
+	}
+	return io.NewSectionReader(f, start, zlen), zlen, true
+}