nx_zstd_frame_gate.nx

nx_zstd_frame_gate.nx -- proves the zstd frame and block layer. T3 pins the 2-BYTE FRAME_CONTENT_SIZE BIAS. A two-byte size stores (size - 256), so a naive reader reports every such frame exactly 256 bytes short. The affected range is 256..65791 bytes, which is where small test files live -- so this bug hides in exactly the frames a developer checks by hand. The test asserts a stored 0 means 256. T2 pins the IRREGULAR field-size tables. Neither FCS nor Dictionary_ID size is the flag value: FCS goes 0/2/4/8 (or 1 when Single_Segment is set with flag 0) and DID goes 0/1/2/4. Treating either as the raw flag mis-sizes the header and every byte after it. T6 proves a real end-to-end decode: a frame of raw and RLE blocks IS a valid zstd file, and this decodes one to exact bytes. T7 proves the honest refusal: a Compressed block returns a DISTINCT code rather than passing undecoded bytes through as if they were output. license_tier: ORIGINAL