Select checked implementations explicitly by target and ABI

src/execution-world/README.md

@@ -113,3 +113,29 @@ Common implementation helpers can still be ordinary generic TypeScript functions
 This does not implement universal runtime type application or legacy constructors.
 The `camino-portable-bindings` check compiles positive/negative typed consumers and
 executes both specializations in Node, Chromium Window, and Worker hosts.
+
+Use `selectArtifactImplementation(checkedManifest, selection)` to choose an
+implementation by target, ABI, required export IDs, and optionally its exact ID.
+Without an ID, exactly one candidate must match. Missing exports, wrong targets,
+and ambiguous choices fail; manifest order never determines execution. This
+selects already checked bytes and does not grant permission to run untrusted
+package code in process. Multiple selected implementations remain part of the
+same checked world's complete artifact inventory.
+
+```ts
+const implementation = selectArtifactImplementation(manifest, {
+  target: "server",
+  compatibility: "quixos-service-v1",
+  id: "isolated-service",
+  exports: requiredExports,
+});
+```
+
+Consumers use the selected entrypoint, registry export and export coverage from
+that manifest. The reader's Nix builder accepts per-package
+`implementationSelection = { "package:reader-runtime@1" = {
+  browser = "browser-module"; server = "server-module"; service = "service";
+}; };` overrides. Its fixture requires one implementation per lane covering that
+lane's needed exports; the selector itself permits a smaller requested subset.
+The installation records the chosen service and module entries. Process lifecycle
+and trusted-module placement still belong to the existing runner.