nx_reader_sync.nx

nx_reader_sync.nx -- R1 SYNC SPINE (beat-Kindle ladder #1; census cells SC01/SC37): per-USER cross-device reading position for the library, behind THE WALL. Whispersync-class resume with a sovereign twist: TEXT books -> AUTO-position: the daemon already serves every chapter fetch behind the wall, so recording "last chapter opened" per (user, book) costs ZERO client changes -- the zero-JS text reader stays zero-JS (census SC59 preserved) and still syncs across devices. COMICS/scans -> EXPLICIT position: the zoom viewer (authed JS by design) POSTs {"page":N} and GETs it back on open, so a phone resumes exactly where the desktop left off. Storage: reader/<slug>/pos_<ukey>.json (client JSON stored VERBATIM, atomic tmp+rename, last-write-wins) + reader/<slug>/auto_<ukey>.txt ("<chapfile>\t<epoch>"). <ukey> = sanitized handle + fnv32-hex8 of the RAW handle (deterministic per user, collision-safe after sanitize, path-safe by construction). The user handle comes from nx_sa_validate_handle at the wall -- identity is the AUTH layer's, never a client claim. FAIL-CLOSED: slug via zs_slug_safe -> 403; body cap 2048 -> 413; auto-record NEVER breaks the book serve (any doubt -> silent skip). Lib (no main); nx_reader_sync_gate drives the handlers in-process. license_tier: ORIGINAL