nx_zstd_bits_gate.nx

nx_zstd_bits_gate.nx -- proves zstd's BACKWARD reader and Huffman weights. T2 pins the SENTINEL. zstd streams carry no length: the final byte's highest SET bit marks the end of the data, and everything above it is padding. The test uses two last bytes with the marker at OPPOSITE ends -- 0x81 (marker at bit 7, so almost nothing is padding) and 0x01 (marker at bit 0, so the whole final byte is consumed by the sentinel). A reader that assumes a fixed padding width passes one and fails the other. T3 is the clean identity: with a final byte of 0x01 the readable data is exactly the preceding bytes, so reading 8 bits must return that byte's own value. Bits are packed LSB-first within a byte but the stream is walked HIGH-to-LOW overall; only getting both right reproduces the byte. T6 pins the INFERRED LAST WEIGHT via KRAFT EQUALITY. zstd never stores the final Huffman weight -- it is whatever completes the set to an exact power of two. The check is that after completion, sum of 2^(tableLog - length) over all present symbols equals 2^tableLog EXACTLY. A decoder that reads only the stored weights builds a table missing its last symbol, decodes most literals correctly, and produces one wrong byte wherever that symbol appears. license_tier: ORIGINAL