Adopt pylsl 1.18.5 numpy pulls and string chunk pushes

[cboulay]

Picks up what the last two pylsl releases (v1.18.4, v1.18.5) make available. Requires pylsl>=1.18.5.

Inlet

Numeric pulls pass as_numpy=True alongside the existing pre-allocated fetch buffer. pylsl returns a view of that buffer trimmed to the samples received plus float64 timestamps, so the manual fetch_buffer[:n] slicing and the np.array(timestamps) round-trip are gone. The trim-copy stays, because the buffer is reused by the next pull.

This also removes a silent dependency on ClockSync._offset being a np.float64: lsl2system() was previously handed a Python list of timestamps and only worked because numpy coerced it.

String pulls stay on the list path deliberately — as_numpy=True returns raw undecoded bytes for string streams, whereas the list path gives decoded str.

Per-pull cost (float32, min_samples=1, 1 s fetch buffer):

stream	before	after
8 ch x 10	16 us	15 us
32 ch x 50	16 us	16 us
128 ch x 100	20 us	16 us
512 ch x 200	50 us	40 us

Dropping the fetch buffer entirely (as_numpy with no dest_obj) also works and is simpler, but pylsl then allocates max_samples x n_ch per pull and concatenates the two-phase halves, so cost scales with local_buffer_dur * fs * n_ch rather than with the data returned — 89 us at 512 ch x 200. Keeping the buffer as dest_obj was the faster option at every width.

Outlet

String streams push in one push_chunk (new in 1.18.5) instead of a push_sample loop. pylsl can't unpack a 2-D array for string streams, so it gets a flat list, with one timestamp per sample to preserve the previous behaviour.

New canonical_channel_format() maps <U*, S* and object dtypes all to "string". Fixed-width unicode arrays previously raised KeyError: '<U3' on outlet creation; normalizing the width out of the format name also keeps source_id stable as the longest value in a stream grows.

Not adopted

Two-phase min_samples pulls and transp_sync_blocking were already in use. 1.18.5's length-delimited string blobs mean values with embedded NULs now survive in both directions for free, and 1.18.4's non-UTF-8 metadata tolerance makes _parse_channel_metadata safer with no change here. reset_uid is outlet-side and would defeat the inlet's uid-based reconnect detection.

Test plan

• pytest — 50 passed, including a new string round-trip through OutletProcessor to a real inlet, and mock-inlet assertions that as_numpy is set on the buffered path only.
• Manual smoke test of the inlet string path end to end (decoded <U values, float64 time axis).