Add native-TS functional toolkit (assertNever, Result, Brand)

The shared foundation for the "make impossible states impossible" work:
- assertNever for compile-time exhaustiveness in union switches
- Result<E,T> + ok/err constructors (plain objects, no classes)
- Brand<T,B> for nominal types

No runtime dependency — this is the whole "library".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/core/fp.ts

@@ -0,0 +1,23 @@
+/**
+ * Tiny native-TS functional toolkit. No dependency — this is the whole "library".
+ * Reused by every "impossible states" concept in the POC.
+ */
+
+/** Exhaustiveness guard: put in the `default` arm of a union switch. Adding a
+    new variant without handling it then fails to compile (x is no longer never). */
+export function assertNever(x: never): never {
+  throw new Error('Unexpected variant: ' + JSON.stringify(x));
+}
+
+/** A computation that either succeeded with a value or failed with an error.
+    Plain objects (no classes) to match the signal/httpResource ergonomics. */
+export type Result<E, T> =
+  | { readonly ok: true; readonly value: T }
+  | { readonly ok: false; readonly error: E };
+
+export const ok = <T>(value: T): Result<never, T> => ({ ok: true, value });
+export const err = <E>(error: E): Result<E, never> => ({ ok: false, error });
+
+/** Nominal typing: Brand<string, 'Postcode'> is assignable from a plain string
+    only through an explicit cast — so a smart constructor is the only minter. */
+export type Brand<T, B extends string> = T & { readonly __brand: B };