feat(server): enable write-stall backpressure by default (--write-stall-l0 24)#95
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write_stall_l0 defaulted to 0 (off), so a writer could outrun compaction and let L0 SST count grow unbounded, inflating read amplification and eventually stalling reads. Default it to 24 (3x the reactive-compaction trigger of 8): invisible under normal load, applies the soft --write-stall-delay only under sustained write overload. Set --write-stall-l0 0 to restore the old unbounded behaviour. Embedded AppState and explicit test configs are unchanged.
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Backpressure was off by default, letting a writer outrun compaction and grow L0 unbounded (read amplification, eventual read stalls). Default --write-stall-l0 to 24 (3x the reactive-compaction trigger): invisible under normal load, only bites under sustained overload. Opt out with --write-stall-l0 0. Behavior change, noted in CHANGELOG. Server tests green, fmt clean.