fix: address Copilot review round 10 — structural const match, deep-link constants

- selectBranchIndex compares a discriminator structurally: a `const` may be
  an object or array, and deep-link arguments arrive as freshly parsed
  instances that could never be reference-equal to the schema's own.
- New applySchemaConstants, applied by InspectorView AFTER the deep link's
  `appArgs` are spread over the seeded defaults. The form renders a pinned
  field read-only, so a link disagreeing with one would otherwise auto-open an
  App with a hidden value contradicting the shape on screen.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/core/rootUnion.test.ts

@@ -695,6 +695,25 @@ describe("resolveRootUnion", () => {
       expect(selectBranchIndex(undiscriminated, {})).toBeNull();
     });
 
+    it("matches an object-valued discriminator structurally", () => {
+      // Deep-link arguments are freshly parsed instances, never `===` the
+      // schema's own constant.
+      const objectPinned = resolveRootUnion({
+        type: "object",
+        anyOf: [
+          {
+            type: "object",
+            properties: { tag: { const: { a: 1 } }, x: { type: "string" } },
+          },
+          {
+            type: "object",
+            properties: { tag: { const: { a: 2 } }, y: { type: "string" } },
+          },
+        ],
+      }).branches;
+      expect(selectBranchIndex(objectPinned, { tag: { a: 2 } })).toBe(1);
+    });
+
     it("reports none when the values identify nothing", () => {
       expect(selectBranchIndex(branches, {})).toBeNull();
       expect(selectBranchIndex(branches, { kind: "other" })).toBeNull();