nx_zstd_fse_dec_gate.nx

nx_zstd_fse_dec_gate.nx -- proves the FSE decoding state machine. T3 is the invariant that matters most: over a long decode the state must ALWAYS stay inside the table and the symbol must always be one the distribution actually contains. An FSE decoder whose newState baseline or nbBits is off produces in-range garbage for a while and then walks out of the table -- so a single decode proves nothing and a long run proves a lot. T4 pins that nbBits of ZERO is legal. High-probability symbols cost no bits to advance from; a decoder that treats zero as an error, or reads a bit anyway, desynchronises on the first such state. The test asserts at least one zero-cost state exists in a skewed distribution and that advancing from it consumes nothing. T5 pins DETERMINISM: the same stream and table must produce the same symbol sequence every time. That is what makes an entropy decoder usable at all. NOT PROVEN HERE, DELIBERATELY: the predefined distributions for literal lengths, match lengths and offsets. Those are spec DATA and are not in the module -- transcribing them from memory would yield a decoder that looks right and disagrees with every other implementation. license_tier: ORIGINAL