feat(tsconnect/wasm): add hasShares filter to listDrivePeers

PeerCapabilityTaildriveSharer says a peer may share with us, not that it
does, so listDrivePeers is a superset of the peers actually exposing
shares. listDrivePeers now takes an options object; with
{hasShares: true} each candidate's taildrive root is probed with a
Depth-1 PROPFIND and only peers listing at least one share are kept.

The probe and its multistatus parsing live in cmd/tsconnect/driveprobe
so they can be tested without syscall/js. Probes run in parallel with a
bounded worker count, a per-probe timeout and a bounded response read;
a probe that fails drops that peer and is logged rather than failing
the call, so the filter is positive-only.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/tsconnect/wasm/drive_stub.go

@@ -20,7 +20,7 @@ func initDriveForRemote(_ *tsd.System) *jsFileSystemForRemote { return nil }
 func wireDriveJS(_ *jsIPN, _ *jsFileSystemForRemote, _ map[string]any) {}
 
 // listDrivePeers returns an empty list when the drive feature is omitted.
-func (i *jsIPN) listDrivePeers() js.Value {
+func (i *jsIPN) listDrivePeers(_ bool) js.Value {
 	return makePromise(func() (any, error) {
 		return "[]", nil
 	})