nx_tar_gate.nx

nx_tar_gate.nx -- proves the USTAR reader/writer by ROUND-TRIP. T4 builds a real two-file archive and walks it back, recovering both names, both sizes and every content byte. Symmetric proof: only a correct pair can pass, since a writer bug and a matching reader bug would still have to agree with the absolute assertions in T1-T3. T2 pins OCTAL, not binary: size 8 must be the ASCII digits "10", because tar's numeric fields are octal text. A binary writer produces headers that look fine in a hex dump and no tar on earth can read. T6 pins the two-block terminator. One zero block is not an archive end; readers scan for two. T7 is the integrity proof: corrupt one header byte and the entry must be REFUSED by checksum, then restored and re-accepted so the test is shown to have measured that byte. NON-VACUITY: T8 covers refusals -- an over-long name, an oversize field, a short output buffer and a bad magic must each be rejected. license_tier: ORIGINAL