nx_zstd_seq_gate.nx

nx_zstd_seq_gate.nx -- proves zstd sequence execution and code tables. T4 is the one that matters. An offset SMALLER than the match length is legal and common -- it is how zstd encodes runs -- and the copy is REQUIRED to read bytes it wrote moments earlier in the same operation. The test uses offset 3 with match length 4 (the fourth byte read is one this copy just wrote) and offset 1 with match length 5 (a pure run). A bulk move that snapshots the source region gives the right answer for the first `offset` bytes and garbage after, so both cases are needed: the offset-1 run fails loudly, the offset-3 overlap fails on exactly one byte. T1 pins the DISCONTINUOUS sequence count: the byte itself below 128, a two-byte form below 255, and an escape to three bytes at exactly 255 biased by 0x7F00. A plain varint reading mis-sizes the header and every table after it. T2/T3 pin the code tables, including that match lengths start at THREE and that the literal-length baselines are the value itself only up to 15. license_tier: ORIGINAL