Own the Linux runtime layout discovery; ship the GTK main-thread bridge

[turinglambdaai]

Closes #246. Closes #247.

Runtime layout discovery (#246)

The staged-vs-packaged layout is Rivet's contract, but its discovery was hand-copied per product (autarx AppModel.swift:255 — an admitted third Swift copy, taskly HostResources.swift, syncpilot discover_runtime_layout, and the Linux host template's own main.cpp). rivet::linux_runtime now owns it (layout.hpp/layout.cpp, part of rivet::platform):

• discover_layout() locates runtime/*.boot + res/core.zo beside the running executable (/proc/self/exe)
• discover_layout(stage_root) explicit-root overload for tests and unusual staging
• RuntimeLayout exposes stage_root, resources, and the four artifact paths
• layout_error_hint() is the remediation message hosts display verbatim
• the template main.cpp consumes the library instead of carrying a private copy

macOS parity note: RivetEmbedding already exposes EmbeddedRacketConfiguration.resolvedDefault with the same stage-root logic; docs/embedding.md is the cross-platform pointer. Products migrate by rebasing on the next release and deleting their private copies.

GTK main-thread bridge (#247)

The host template ships src/rpc_bridge.hpp: unpack flattens the generated Result<T> — including the Result<void> specialization Void RPCs produce — into Unpacked<T> (value or error, decided on the reader thread, no exception flow across the hop); post_to_main runs the closure on the GTK main loop, heap-allocated on the reader thread and freed there. The template main.cpp drops its private IntResult/g_idle_add plumbing and shows the pattern end to end. platform/linux/host/README.md documents both under "Runtime layout discovery" and "Completions belong on the main loop".

Verification

• RivetLayoutTest joins the Linux integration CTest suite (complete/truncated/empty layouts, hint content, executable-path resolution)
• layout_test.cpp compiled and run locally (non-GTK paths); rpc_bridge.hpp template instantiations (unpack<int> success/error, unpack<void> against the generator's own Result<void> shape) compiled and run locally with glib; main.cpp's GTK link compiles in the Linux CI legs
• rivet_linux_sources.cmake adds layout.cpp to the rivet::platform library target; downstream hosts link the target and never compile Rivet sources themselves (issue Host integrations must consume released rivet packages, never compile rivet sources into downstream executables #238 contract)