linux observer

feat(contract): adopt observer-client bundle 8.0.0 master

Re-vendor the journal-owned observer-client contract byte-for-byte from solstone-journal 766021cd44d4a0a7ce471d2affb461bf3ce0fc39. Replace the five files under vendor/observer-client-contract/ and move provenance, Rust contract-test pins, Makefile gate echoes, policy-test literals, and contracts/README.md to 8.0.0. The bundle drops the chat.openSolChatRequest operation (4 fixtures, 2 vectors). LINUX_FIXTURES and LINUX_VECTORS are unchanged.