perf(info): avoid duplicate desc().child() lookup in _get_channel_info

[sappelhoff]

(PR and content generated with the help of Claude)

What

Compute self.desc().child("channels") once in StreamInfo._get_channel_info instead of twice.

Why

The method fetched the same node twice — once for the .empty() guard and again to start iterating:

if self.desc().child("channels").empty():
    return None
ch_infos = list()
channels = self.desc().child("channels")

Each self.desc().child("channels") is two liblsl FFI calls (lsl_get_desc + lsl_child) plus an XMLElement allocation. Now it's computed once and reused:

channels = self.desc().child("channels")
if channels.empty():
    return None
ch_infos = list()

Impact

Eliminates 2 redundant FFI calls + 1 object allocation per call to get_channel_labels/get_channel_types/get_channel_units. This is a cold path (metadata read, not streaming), so the runtime impact is small — it's mainly a clarity/cleanliness fix.

Behavior

Unchanged. Verified the empty->None branch and label/type/unit round-trips; existing test suite passes.