Adding a Zelda3-style compare lane to the TD2 port

Adding a Zelda3-style compare lane to the TD2 port The SDL port now has its first built-in compare/reporting path instead of relying only on ad hoc external frame diffs. This checkpoint adds --compare to the native runtime. For a promoted design-pack fixture, the binary now emits: the native runtime frame the trusted main_visible.ppm golden an absolute RGB diff map a runtime | golden | diff strip a JSON summary with mismatch, ratio, max channel delta, mean absolute error, and RMSE The current cheap validation surface is now: make -C port test That covers both: exact frame parity on the promoted fixtures compare-bundle generation on the same fixtures The promoted compare fixtures are still exact: frame 300: 0 mismatched pixels frame 1086: 0 mismatched pixels The immediate value is not the pretty strip by itself. The important part is that the runtime now has a built-in place where callback/state drift can be reported once the callback execution spine is promoted into C. I also added a r