Only register shms w sampler when `feed_is_live`

Add timeout-gated wait for `feed_is_live: trio.Event` before passing shm
tokens to `open_sample_stream()`; skip registering shm-buffers with the
sampler if the feed doesn't "go live" within a new timeout.

The main motivation here is to avoid the sampler incrementing shm-array
bufs when the mkt-venue is closed so that a trailing "same price"
line/bars isn't updated/rendered in the chart's view when unnecessary.

Deats,
- add `wait_for_live_timeout: float = 0.5` param to `manage_history()`
- warn-log the fqme when timeout triggers
- add error log for invalid `frame_start_dt` comparisons to
  `maybe_fill_null_segments()`.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/tsp/_history.py

@@ -247,6 +247,11 @@ async def maybe_fill_null_segments(
                 from_timestamp(array['time'][0])
             ) < backfill_until_dt
         ):
+            log.error(
+                f'Invalid frame_start !?\n'
+                f'frame_start_dt: {frame_start_dt!r}\n'
+                f'backfill_until_dt: {backfill_until_dt!r}\n'
+            )
             await tractor.pause()
 
         # XXX TODO: pretty sure if i plot tsla, btcusdt.binance
@@ -1284,6 +1289,7 @@ async def manage_history(
     some_data_ready: trio.Event,
     feed_is_live: trio.Event,
     timeframe: float = 60,  # in seconds
+    wait_for_live_timeout: float = 0.5,
 
     task_status: TaskStatus[
         tuple[ShmArray, ShmArray]
@@ -1432,12 +1438,26 @@ async def manage_history(
             1: rt_shm,
             60: hist_shm,
         }
-        async with open_sample_stream(
-            period_s=1.,
-            shms_by_period={
+
+        shms_by_period: dict|None = None
+        with trio.move_on_after(wait_for_live_timeout) as cs:
+            await feed_is_live.wait()
+
+        if cs.cancelled_caught:
+            log.warning(
+                f'No live feed within {wait_for_live_timeout!r}s\n'
+                f'fqme: {mkt.fqme!r}\n'
+                f'NOT activating shm-buffer-sampler!!\n'
+            )
+
+        if feed_is_live.is_set():
+            shms_by_period: dict[int, dict] = {
                 1.: rt_shm.token,
                 60.: hist_shm.token,
-            },
+            }
+        async with open_sample_stream(
+            period_s=1.,
+            shms_by_period=shms_by_period,
 
             # NOTE: we want to only open a stream for doing
             # broadcasts on backfill operations, not receive the