Add buffer capacity checks to backfill loop

Prevent `ValueError` from negative prepend index in
`start_backfill()` by checking buffer space before push
attempts. Truncate incoming frame if needed and stop gracefully
when buffer full.

Also,
- add pre-push capacity check with frame truncation logic
- stop backfill when `next_prepend_index <= 0`
- log warnings for capacity exceeded and buffer-full conditions

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

piker/tsp/_history.py

@@ -577,6 +577,25 @@ async def start_backfill(
                     f'{next_start_dt} -> {last_start_dt}'
                 )
 
+            # Check if we're about to exceed buffer capacity BEFORE
+            # attempting the push
+            if next_prepend_index - ln < 0:
+                log.warning(
+                    f'Backfill would exceed buffer capacity!\n'
+                    f'next_prepend_index: {next_prepend_index}\n'
+                    f'frame size: {ln}\n'
+                    f'Truncating to fit remaining space..\n'
+                )
+                # only push what fits
+                to_push = to_push[-(next_prepend_index):]
+                ln = len(to_push)
+
+                if ln == 0:
+                    log.warning(
+                        'No space left in buffer, stopping backfill!'
+                    )
+                    break
+
             # bail gracefully on shm allocation overrun/full
             # condition
             try:
@@ -597,6 +616,14 @@ async def start_backfill(
                 next_prepend_index = next_prepend_index - ln
                 last_start_dt = next_start_dt
 
+                # Stop if we've hit buffer start
+                if next_prepend_index <= 0:
+                    log.warning(
+                        f'Reached buffer start (index={next_prepend_index}), '
+                        f'stopping backfill'
+                    )
+                    break
+
             except ValueError as ve:
                 _ve = ve
                 log.error(