fix: address Copilot review — const precedence, TUI variant model, decline both-keyword unions

- rootUnion: decline a schema carrying BOTH oneOf and anyOf rather than
  reading one and silently dropping the other's constraints.
- SchemaForm: handle `const` before every type dispatch and for every
  constant type, so a numeric/boolean constant, or a schema carrying both
  `const` and `enum`, can no longer offer a forbidden value.
- collectSchemaDefaults / TUI initialValue: `const` outranks `default`,
  which is an annotation a schema may set to something its const rejects.
- convertToolParameters: resolve from the first branch's merged schema, so a
  branch's specialization of a root-declared property is what types the arg.
- TUI: ink-form keys values by field name across the whole form, so two
  branches' `kind` were one field and the later section's initial value
  decided what the earlier submitted. Branch fields now render under prefixed
  names behind a Variant select, and decodeFormValues translates back on
  submit, dropping every branch but the chosen one.

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

clients/web/src/utils/jsonUtils.test.ts

@@ -382,7 +382,10 @@ describe("root composition (#2123)", () => {
     ).toEqual({ version: "1" });
   });
 
-  it("prefers an explicit default over a const", () => {
+  it("prefers a const over a conflicting default", () => {
+    // `default` is an annotation, not a constraint, so a schema may advertise
+    // one its own `const` rejects — seeding it would submit an invalid value
+    // through a read-only field.
     expect(
       collectSchemaDefaults({
         type: "object",
@@ -390,7 +393,7 @@ describe("root composition (#2123)", () => {
           v: { type: "string", const: "a", default: "b" },
         },
       }),
-    ).toEqual({ v: "b" });
+    ).toEqual({ v: "a" });
   });
 
   it("collects defaults from a root allOf", () => {

clients/web/src/utils/jsonUtils.ts

@@ -139,13 +139,19 @@ export function collectSchemaDefaults(
     // without this the form would *display* a hoisted default that never
     // reached the seeded values — the field would submit empty (#1928).
     const fieldSchema = normalizeNullableUnion(rawSchema);
-    if (fieldSchema.default !== undefined) {
-      result[fieldName] = fieldSchema.default;
-    } else if (fieldSchema.const !== undefined) {
+    if (fieldSchema.const !== undefined) {
       // `const` is a one-value enumeration, so the only submittable value is
       // already known — seeding it spares the user hand-typing a discriminator
-      // the schema has fixed (#2123), and matches what a `default` would do.
+      // the schema has fixed (#2123).
+      //
+      // It outranks `default`, which JSON Schema defines as an annotation
+      // rather than a constraint: a schema may advertise a default its own
+      // `const` rejects, and seeding that would submit an invalid argument
+      // through a field rendered read-only, leaving the user no way to correct
+      // it.
       result[fieldName] = fieldSchema.const;
+    } else if (fieldSchema.default !== undefined) {
+      result[fieldName] = fieldSchema.default;
     } else if (fieldSchema.type === "object" && fieldSchema.properties) {
       const nested = collectSchemaDefaults(fieldSchema);
       if (Object.keys(nested).length > 0) {