P21 adds the official AUIPC test

P21 widens P20 from twenty-two official tests to twenty-three by adding I-auipc-00.S.

Command:

make -C projects/21_rv32i_auipc_tests/test

Result: PASS

Summary: PASS=23 FAIL=0 NOT RUN=0

The interesting bit was not the core change; there was no RTL change. The signature model needed to handle PC-relative expectations. The generated signature file now uses label expressions so the linker supplies the real instruction addresses.

Next likely target: JAL, then JALR, branches, and memory-access tests.