Record file mode in the manifest, 0000 unless asked (closes #161)

MFFilePath gains mode (field 304): a file's permission bits, 0777 at
most, or 0000, meaning none recorded. gen and freshen record real modes
only with --include-permissions (ScannerOptions.IncludePermissions); the
builder keeps only mode.Perm(), so setuid, setgid and sticky are never
written. list -l and export show the mode in octal. check reports
MODE_MISMATCH for a recorded mode other than 0000 the file lacks. fetch
refuses a manifest with a mode above 0777 before requesting any file,
sets only the permission bits of each recorded mode on the files it
writes, and downloads again a present file whose mode differs. The decoding-cost bound counts a file
entry at 176 bytes, up from 160.

Model: opus-5-5

mfer/builder.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"sort"
 	"strings"
 	"sync"
@@ -137,12 +138,14 @@ func (b *Builder) SetSeed(seed string) {
 }
 
 // AddFile reads file content from reader, computes hashes, and adds to manifest.
+// Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
 // Progress updates are sent to the progress channel (if non-nil) without blocking.
 // Returns the number of bytes read.
 func (b *Builder) AddFile(
 	path RelFilePath,
 	size FileSize,
 	mtime ModTime,
+	mode fs.FileMode,
 	reader io.Reader,
 	progress chan<- FileHashProgress,
 ) (FileSize, error) {
@@ -198,6 +201,7 @@ func (b *Builder) AddFile(
 			{MultiHash: mh},
 		},
 		Mtime: mtime.Timestamp(),
+		Mode:  uint32(mode.Perm()),
 	}
 
 	b.mu.Lock()
@@ -229,12 +233,14 @@ func (b *Builder) FileCount() int {
 
 // AddFileWithHash adds a file entry with a pre-computed hash.
 // This is useful when the hash is already known (e.g., from an existing manifest).
+// Only mode's permission bits (mode.Perm()) are recorded; 0 records none.
 // Returns an error if path is invalid, size is negative, or hash is not a
 // multihash with a digest of at least 32 bytes, as long as SHA-256's.
 func (b *Builder) AddFileWithHash(
 	path RelFilePath,
 	size FileSize,
 	mtime ModTime,
+	mode fs.FileMode,
 	hash Multihash,
 ) error {
 	err := ValidatePath(string(path))
@@ -268,6 +274,7 @@ func (b *Builder) AddFileWithHash(
 			{MultiHash: hash},
 		},
 		Mtime: mtime.Timestamp(),
+		Mode:  uint32(mode.Perm()),
 	}
 
 	b.mu.Lock()