[mstan]

A core cannot adapt its native widescreen projection when the frontend resizes because the runner receives only pads. Add an optional rcore_host_api::presentation_size callback and carry the available drawable pixel area in each CoreLink grant, before aspect letterboxing. The runner freezes that area for the grant, including resims; headless or unavailable dimensions return zero.

The append-only contract becomes rcore draft 9 / CoreLink 2.3. The callback is guarded by host struct_size; the grant retains its 240-byte 2.2 prefix and sends that prefix to older peers. Shared-memory frame layout and protocol major stay unchanged. This is local presentation policy, outside guest input and serialized simulation state.

Validation: MSVC Release build; all 32 CTests passed, including resize/unavailable/recovery callback checks and old-prefix decoding. Actual 2.2 host with 2.3 runner and 2.3 host with 2.2 runner both completed successfully. Independent C layout measurement: host API size 160, callback offset 152.

Prerequisite for the dependent Retro-Launcher adaptive presentation PR and n64lle Fit consumer. Merge this first; no release or merge is requested here.
Related adaptive Smash stack: RetroPortingToolKit/n64lle#97 and RetroPortingToolKit/SuperSmashBrosRecomp#1. Merge Runtime #7 first, then Launcher #35 with its Runtime pin preserved or updated to the merged commit; merge n64lle #97 before the game PR. No merge or release has been performed by the agent.