nx_livemap_fast.nx

nx_livemap_fast.nx -- LINEAR live-doc map construction, to replace the quadratic one in ss_open2. THE PROBLEM (nx_seg_store.nx:1739-1790): for every key in every segment, the current code scans EVERY NEWER SEGMENT calling ss_idx_find to ask `has a later segment shadowed this key`. That is O(keys x segments) index probes -- tens of millions on the live 94-segment web shard -- and it is QUADRATIC IN SEGMENT COUNT, so it degrades every time the crawler commits. It is the measured cause of the ~8 minute docportal pre-warm and, through that, of tonight's search wedge. THE INSIGHT: the shadow question is `does any NEWER segment know this key`. Answering it per-key by rescanning newer segments repeats the same work once per key. Walk the segments ONCE from NEWEST to OLDEST instead, recording the first (= newest) segment that knows each key. Then a key in segment s is shadowed iff its recorded newest segment is greater than s. One pass, one probe. NOTHING HERE IS WIRED INTO PRODUCTION. This file only ADDS functions; ss_open2 is untouched. The companion gate proves this produces BYTE-IDENTICAL maps to the existing implementation on the real shard before anyone considers swapping. A faster live-doc map that is subtly different does not make search quick, it makes search WRONG -- these maps decide which documents are visible at all. license_tier: ORIGINAL No hw writes (Rule 26).