ktsaou
An unaligned RAM/ALLOC fine ring can discard the last row of a retained coarse island after the disconnected-lookbehind clamp in #24097. This narrow regression remains deferred under the approved surgical scope; it needs a separate correction and regression fixture. At production head `b5934b3784`, use a 5-second collector with timestamps at `5k+3`. ALLOC reports the oldest slot's start, and the ring starts 3 seconds after the coarse island ends. The clamp picks this slot start; the first empty fine placeholder begins 2 seconds before the island end, so read-ahead truncates the coarse contribution. The supplied comparative probes show row `(580,600]`: - AVERAGE: current head NULL; preceding head and master 7. - SUM: current head 25.2; master 28, which is the fixture truth. The preceding head has a separate loss. - Across the tested geometry: 14 AVERAGE/MAX rows become empty and 7 SUM rows lose 2/60 of a record. Relevant paths: - [Disconnected fine lookbehind](https://github.com/netdata/netdata/blob/b5934b3784d77f0f742e46652ca4dd1a5e8c5649/src/web/api/queries/query-plan.c#L248-L254). - [Read-ahead horizon and interval overlap](https://github.com/netdata/netdata/blob/b5934b3784d77f0f742e46652ca4dd1a5e8c5649/src/web/api/queries/query-execute.c#L508-L534). - [ALLOC slot boundaries](https://github.com/netdata/netdata/blob/b5934b3784d77f0f742e46652ca4dd1a5e8c5649/src/database/rrddim_mem.c#L604-L607). Reach requires non-default RAM/ALLOC tier 0, unaligned timestamps and a fine ring beginning within one fine interval of the island end. Native collectors align timestamps; external streaming producers can supply the unaligned shape. Frequency has not been measured. Acceptance: - Add an independently keyed coarse-island boundary regression for the unaligned shape, including AVERAGE/MAX and retained SUM volume. - Restore the coarse island's last row and preserve the first retained fine point. - Compare aligned, disconnected, connected-overlap and ring-start-at-island-end controls. - Preserve the approved tier fallback order and genuine gap behavior.