[Dimoshka]

Implements the two remaining checklist items of #39.

Item A — pathContext seam for ServerBinaryLocatorImpl

ServerBinaryLocatorImpl now takes an optional p.Context? pathContext (default p.context) and renders every join, normalize, dirname, isAbsolute and PATH-list separator through it, mirroring LlamaLibraryLocator. Production defaults and the executability logic are unchanged. New tests cover:

• a Windows context (p.Style.windows): C:\Program Files\LlamaPulse\data\llama-server.exe, the adjacent llama-server.exe, the config bin candidate and the PATH entry with the bare .exe name last;
• a POSIX context: <X.app>/Contents/Resources/runtime/v0.8.0/metal/llama-server is produced and resolved.

Item B — CI verifies a runtime-bearing macOS bundle

The Package (macOS) job keeps the runtime-free package/upload path and adds:

1. a deterministic fixture runtime in $RUNNER_TEMP — a compiled Mach-O libllama.dylib, a libggml-metal.dylib copy, an executable llama-server script and a realistic runtime.yaml;
2. package_app.dart --runtime v0.8.0 metal --out dist-runtime;
3. ditto -x -k, then asserts the four runtime files under Contents/Resources/runtime/v0.8.0/metal/, the app executable bit, codesign --verify --deep --strict, and no com.apple.security.app-sandbox entitlement.

docs/packaging.md now says CI packages a runtime-bearing macOS bundle too, but with a fixture runtime, not a downloaded production archive.

Still unverified

• A downloaded production Metal runtime and real GPU backends.
• Windows FFI loading of a bundled runtime.
• A GUI launch, model load or any runtime execution.

Test plan

• dart run melos run check green: gen twice with no drift, gen:catalog:check (362 parameters), analyze, 1293 tests (894 core plus two host/env-gated skips, 233 CLI, 166 app), format:check, CLI AOT build.
• CI on this PR (all seven jobs) exercises the new macOS runtime-bearing leg.

Refs #39