feat: vendor bodgit/sevenzip package to remove go4.org dependency

Vendored the sevenzip package to eliminate dependency chain:
- sevenzip -> go4.org -> 25+ Google/Cloud/telemetry packages

Changes:
- Added internal/sevenzip/ with full package source
- Inlined go4.org/readerutil into multireaderat.go
- Updated all internal imports to use bookhoard/internal/sevenzip
- Preserved .cb7 comic archive support

This reduces bloat by ~4.9 MB and removes unused telemetry
dependencies while maintaining all functionality.

internal/sevenzip/multireaderat.go

@@ -0,0 +1,87 @@
+// Package sevenzip provides read access to 7-Zip archives.
+//
+// Portions Copyright 2016 The go4 Authors
+// Licensed under the Apache License, Version 2.0
+package sevenzip
+
+import (
+	"io"
+	"sort"
+)
+
+// SizeReaderAt is a ReaderAt with a Size method.
+//
+// An io.SectionReader implements SizeReaderAt.
+type SizeReaderAt interface {
+	Size() int64
+	io.ReaderAt
+}
+
+// NewMultiReaderAt is like io.MultiReader but produces a ReaderAt
+// (and Size), instead of just a reader.
+func NewMultiReaderAt(parts ...SizeReaderAt) SizeReaderAt {
+	m := &multiRA{
+		parts: make([]offsetAndSource, 0, len(parts)),
+	}
+	var off int64
+	for _, p := range parts {
+		m.parts = append(m.parts, offsetAndSource{off, p})
+		off += p.Size()
+	}
+	m.size = off
+	return m
+}
+
+type offsetAndSource struct {
+	off int64
+	SizeReaderAt
+}
+
+type multiRA struct {
+	parts []offsetAndSource
+	size  int64
+}
+
+func (m *multiRA) Size() int64 { return m.size }
+
+func (m *multiRA) ReadAt(p []byte, off int64) (n int, err error) {
+	wantN := len(p)
+
+	// Skip past the requested offset.
+	skipParts := sort.Search(len(m.parts), func(i int) bool {
+		// This function returns whether parts[i] will
+		// contribute any bytes to our output.
+		part := m.parts[i]
+		return part.off+part.Size() > off
+	})
+	parts := m.parts[skipParts:]
+
+	// How far to skip in the first part.
+	needSkip := off
+	if len(parts) > 0 {
+		needSkip -= parts[0].off
+	}
+
+	for len(parts) > 0 && len(p) > 0 {
+		readP := p
+		partSize := parts[0].Size()
+		if int64(len(readP)) > partSize-needSkip {
+			readP = readP[:partSize-needSkip]
+		}
+		pn, err0 := parts[0].ReadAt(readP, needSkip)
+		if err0 != nil {
+			return n, err0
+		}
+		n += pn
+		p = p[pn:]
+		if int64(pn)+needSkip == partSize {
+			parts = parts[1:]
+		}
+		needSkip = 0
+	}
+
+	if n != wantN {
+		err = io.ErrUnexpectedEOF
+	}
+	return
+}