nx_readcap_lib.nx

nx_readcap_lib.nx -- THE ONE read-into-a-caller's-cap helper (dataio DI16, first cause, 2026-09-06). MEASURED before this lib existed: the io-envelope census worklist held 1,082 silent readers in 803 files, and its SHAPES table showed the top of the population was ONE helper family copied by hand -- `sys_read(fd, buf+n, cap-n)` in a loop that stops when the cap fills and never says so (x28, x22, x20, x19, x15, x13 ...). Every copy is a silent-truncation cap: the caller cannot tell "the file was exactly cap bytes" from "the cap filled and the tail is gone". This lib is the bus stop: ONE loop, ONE named state per outcome, so a consumer that adopts it has no sys_read( site of its own and the census counts it CONVERTED. States (rc_fill): RC_EOF the source ended before the cap (buf holds all of it) / RC_CAPPED the cap filled and the source was NOT probed further (a socket must not be probed: the next byte may never come) / RC_ERR a read failed (st[1] = the rc). rc_read_file_into adds the file-only refinement: after a CAPPED fill it reads ONE more byte, so RC_FULL means "TRUNCATED, more remains" and RC_EXACT means "the file was exactly cap bytes" -- the distinction the hand-rolled loops could never make. rc_state_name spells the TRUNCATED state so any consumer that prints it announces its envelope in the census's own vocabulary. Whole-file reads with no cap at all belong to sys_read_file (nx_syscalls), which sizes from the file; this lib is for the case where the BUFFER is the caller's and the cap is real (a fixed header, a bounded request line, a window). license_tier: ORIGINAL No hw writes (Rule 26).