refactor: Route every input state mutation through one untracked gateway

The untrack()/persistAndProcess() pair was a per-function convention:
each update function had to remember both, a forgotten untrack would let
calling effects subscribe to input state (the bug fixed in 6a9a306e),
and a forgotten persistAndProcess would silently skip persistence and
re-validation. A single update(mutate) gateway now makes both structural,
and the scattered untrack calls (three of which were dead) are gone.

Also:
- Export inputState as Readonly<State> so writes outside the update
  functions are type errors instead of comment violations.
- Share the 'codeStore' key as a constant instead of two literals.
- Reuse validatedStateOf() in processState instead of duplicating the
  validated-state defaults.
- Use $state.raw for validatedCurrent: it is only ever replaced
  wholesale, so deep proxying every published state was pure overhead.

The new state.svelte.test.ts pins the invariants: update functions never
make a calling effect track input state, and every mutation persists.
Vitest needed resolve.conditions=['browser'] for those tests — it was
loading Svelte's server build, where $effect is a no-op.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

vite.config.js

@@ -33,6 +33,8 @@ export default defineConfig({
   envPrefix: 'MERMAID_',
   server: { port: 3000, host: true },
   preview: { port: 3000, host: true },
+  // Vitest otherwise resolves Svelte's server build, where $effect is a no-op.
+  resolve: process.env.VITEST ? { conditions: ['browser'] } : undefined,
   test: {
     environment: 'jsdom',
     // in-source testing