Build Camino packages from explicit contract inputs

Project source-free capability contracts and keep exact source provenance in package assembly. Reuse compiled groups across unrelated graph and dependency implementation edits. Keep React public inputs separate from workspace query lowering and retain explicit type-companion dependencies.

Allow independent execution-package checks, scope resource compilation to declared dependencies, and memoize source closure traversal. Document generic-specialization limits and measured Reader rebuild behavior.

Verified protocol, artifact, package, React, and managed activation checks; retained-Git independent/workspace artifact equality and incompatible type dependency rejection; nix fmt -- --check and lint. One-off Reader measurements recompiled 1 of 13 groups for a child edit and 0 for a workspace QX default edit.

PORTABLE_ARTIFACTS.md

@@ -8,9 +8,9 @@ receipts. It has no migration step or package-defined shell hooks.
 ```nix
 # Inside a per-system flake output:
 quixosPackages.checkedArtifacts =
-  { plan, generator, core, sharedRuntime, typescript, packageRevisionId }:
+  { buildContract, sourceDigest, generator, core, sharedRuntime, typescript, packageRevisionId, ... }:
   helpers.mkCaminoArtifacts {
-    inherit pkgs plan core sharedRuntime typescript packageRevisionId;
+    inherit pkgs buildContract sourceDigest core sharedRuntime typescript packageRevisionId;
     protocol = generator;
     src = ./.;
     portable = {
@@ -22,6 +22,15 @@ quixosPackages.checkedArtifacts =
   };
 ```
 
+`buildContract` is a canonical, source-free capability slice. `sourceDigest`
+records the exact selected package source in the final installation manifest;
+it does not enter the reusable compilation group. Ordinary source dependencies,
+SDKs and React props companions remain explicit Nix inputs. Raw QX and resource
+lock files enter through the checked contract and evaluated recipe, and are
+excluded from the implementation source tree. The `plan` argument on the low-level
+helper is a composition convenience for fixtures; it is projected before building.
+Package recipes receive the contract directly and can be checked without a workspace.
+
 The handler entry exports `registry = createRegistry(handlers)` from the generated
 bindings. One registry contains many exports. The builder always supplies a server
 implementation for the generated process service. A service-only package instead