Add pseudo-instruction support (li, mv, j, ret, etc.) #2
Description
The example programs use pseudo-instructions like li, mv, j, ret but the assembler chokes on them. These are just conveniences that expand to real instructions. Need li (load immediate, expands to lui + addi), mv (move register, just addi rd, rs, 0), j (jump, becomes jal x0, offset), ret (return, jalr x0, ra, 0), nop (addi x0, x0, 0), call/tail (function calls using auipc + jalr), not (xori rd, rs, -1), neg (sub rd, x0, rs), seqz/snez (set if zero/not zero).
The tricky part is li because you need to split 32-bit immediates into upper 20 bits (lui) and lower 12 bits (addi). Once this works the example programs should actually assemble.