Remove unnecessary type assertions across codebase

Add oxlint-tsgolint and configure typescript/no-unnecessary-type-assertion
to flag redundant `!` and `as Foo` casts. Type-aware mode is left off by
default to keep `npm run lint` fast; the rule sits configured for when we
turn type-aware on intentionally. Auto-fix removed ~283 redundant casts;
two manual touch-ups: a real tsgolint false positive in split-container.tsx
and a stale ChildProcess import after a double-cast collapsed.

packages/expo-two-way-audio/examples/flow-api/App.tsx

@@ -91,7 +91,7 @@ export function FlowTest() {
       for (const result of data.results as {
         alternatives: Array<RecognitionAlternative>;
       }[]) {
-        for (const alternative of result.alternatives as Array<RecognitionAlternative>) {
+        for (const alternative of result.alternatives) {
           console.log(alternative.content);
         }
       }