Revert to checking a `.hist` suffix in shm file name

That is, in-mem history buffer files should have a `.hist` suffix (which
was added back instead of the weird `.h` change from the rebased
8352d89e).

I'm pretty sure we can afford the 3 extra chars if we're already
truncating to a max 16 char `key: str = f'{uuid[:8]}_{fqme_hash}'`, per
@goodboy's edit from the original key-truncation approach in the
original original @dnks patch: 75d8582b).

piker/ui/_display.py

@@ -214,9 +214,8 @@ async def increment_history_view(
     hist_chart: ChartPlotWidget = ds.hist_chart
     hist_viz: Viz = ds.hist_viz
     # viz: Viz = ds.viz
-    # NOTE: Changed for macOS compatibility with shortened shm names
-    # assert 'hist' in hist_viz.shm.token['shm_name']
-    assert hist_viz.shm.token['shm_name'].endswith('.h')
+    # Ensure the "history" shm-buffer is what's reffed.
+    assert hist_viz.shm.token['shm_name'].endswith('.hist')
     # name: str = hist_viz.name
 
     # TODO: seems this is more reliable at keeping the slow