Return only the type bits from iso9660 DirEntry.Type

[youdie006]

iso9660's DirEntry.Type() returns the full mode instead of the type bits, so a caller comparing
against fs.ModeDir never matches and reads directories as regular files.

io/fs, fs.go:112:

Type returns the type bits for the entry. The type bits are a subset of the usual FileMode bits,
those returned by the FileMode.Type method.

filesystem/iso9660/directoryentry.go:598:

func (de *directoryEntry) Type() os.FileMode {
	return de.Mode()
}

de.Mode() returns os.ModeDir | 0o755, os.ModeSymlink | 0o777, 0o755, or the Rock Ridge POSIX
mode — permissions included.

Its three siblings already do it right

squashfs/directoryentry.go:77	return d.Mode().Type()
fat12/directoryentry.go:84	iofs.ModeDir / 0
ext4/directoryentry.go:210	type constants only
iso9660/directoryentry.go:599	return de.Mode()

What it costs

testing/fstest.TestFS on the committed testdata/9660.iso:

ABC: mismatch:
	entry = ABC IsDir=true Type=drwxr-xr-x
	file.Stat() = ABC IsDir=true Type=d---------
ABC: Open+ReadAll: invalid argument
BAR: mismatch:
	entry = BAR IsDir=true Type=drwxr-xr-x
	file.Stat() = BAR IsDir=true Type=d---------
BAR: Open+ReadAll: invalid argument
DEEP: mismatch:

Five mismatches, none after the change. The Open+ReadAll: invalid argument lines are the real
cost: fstest dispatches on info.Type() ($(go env GOROOT)/src/testing/fstest/testfs.go:159),
and because drwxr-xr-x is not fs.ModeDir every subdirectory falls to the default branch and is
opened as a file, so the walk never descends. The same run goes from 0.00s to 4.92s with the fix —
it is actually traversing the tree now. Any consumer switching on DirEntry.Type(), or testing
== fs.ModeSymlink to spot a Rock Ridge symlink, hits the same wall. fs.WalkDir survives only
because it uses IsDir().

Change

One line. The check goes into the repo's own shared validator (filesystem/internal/testutil/fsvalidate.go),
which all four filesystem packages already run, so it guards every backend rather than just this one:

// iofs.DirEntry.Type returns only the type bits, not the permissions
if typ := e.Type(); typ != typ.Type() {
    t.Fatalf("entry %q in %q: Type() = %v, want only type bits %v", name, path, typ, typ.Type())
}

Red with only directoryentry.go reverted — and only iso9660 fails:

fsvalidate.go:37: entry "ABC" in ".": Type() = drwxr-xr-x, want only type bits d---------

Green with the change. go test ./filesystem/... passes across all seven packages (ext4 included,
115s), gofmt -l lists neither file, go vet is clean.

This is the shape of #357 ("fix(iso9660): Open() now handles directories and implements
fs.ReadDirFile"), which is also where TestFSTree came from.

One thing I found and did not fix

With this change in place, fstest.TestFS surfaces a second, unrelated violation: a path containing
a backslash resolves to a real entry. Open("DEEP\\A") correctly reports "file does not exist", but
Open("DEEP\\A/B") succeeds — the path-table lookup at filesystem/iso9660/pathtable.go:152 misses
and falls through to rootDir.getLocation, which accepts the bogus component. Separate PR; happy to
send it if useful.

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Disclosure: prepared with AI assistance; I verified the fstest.TestFS output, the sibling
comparison and the red/green runs myself.