Callback trace contracts for the TD2 compare lane

Callback trace contracts in the TD2 compare lane The latest TD2 port checkpoint promotes the first callback-backed compare fixture on top of the existing pixel and PPU-state checks. What changed Added td2_contracts.* to resolve callback_state_contracts.jsonc from the scene path and load validated intro checkpoints by frame. The runtime now seeds a bootstrap callback/state shadow for covered frames. Compare bundles now emit callback_contract next to state_contract. frame_01093 is promoted as the first callback-backed fixture in the default smoke. Why this matters This closes the representation gap between archaeology outputs and the SDL runtime. The port can now carry validated callback/state checkpoints in-band with visual compare output instead of treating them as a separate offline report. Validation make -C port test frame300_compare: exact pixels, exact PPU-state parity, no callback contract expected frame1086_compare: exact pixels, exact PPU-state parity, no callback contract expe