Phase 0c - Structural seams (DONE)

Put the crate boundaries in place before there is code to restructure. Five pieces, from D029-D035 and D048:

• Split the graphics work. orbistoun-gpu (translation, no ash dependency at all) and orbistoun-gpu-vulkan (the only crate that knows Vulkan exists). Makes leaking the host API into the translator impossible rather than merely discouraged.
• orbistoun-service. Extract the logic orbistoun-cli currently owns - build_registry, the survey flow assembled in main.rs - into the shared layer every shim calls. Gets more expensive with every command added, so do it now.
• orbistoun-proto. Shim-to-worker message types as serde data, defined separately from any transport.
• orbistoun-paths. Portable-first resolution per D038. Needed before the first thing that writes, and that is the GUI's settings at phase 2b - earlier than the traces at phase 4.
• orbistoun-overrides. Per-title settings per D048: one file per title, keyed by content hash, three layers merged per key. Structural for the same reason as the gpu split - the mechanism has to exist before the first title-specific need arises, or somebody hardcodes it and the pattern is set.

Observable result: cargo tree shows orbistoun-gpu with no path to ash; the portable containment test passes; the CLI is a shim over the service rather than a holder of logic; an override file changes behaviour with nothing title-specific anywhere in the core.