prepare.cppm: extract P5 (toolchain decision after the graph) as a phase function

P5 decides whether the graph supplies a target row's system, folds a
dependency's compiler requirement into tcSpec, and refuses the
convention/capability conflicts (#704's target matrix). No new
cross-phase state: it only reads members P0-P4 already promoted and
calls state.resolve_target_toolchain(). Zero compile errors on first
extraction.

Golden fixture diff against the baseline is empty; mcpp test: 129/129.

Author: speak-agent