[compiler] `@enablePreserveExistingMemoizationGuarantees` on by default

This enables `@enablePreserveExistingMemoizationGuarantees` by default. As of the previous PR (#34503), this mode now enables the following behaviors:

- Treating variables referenced within a `useMemo()` or `useCallback()` as "frozen" (immutable) as of the start of the call. Ie, the compiler will assume that the values you reference are not mutated by the body of the useMemo, not are they mutated later. Directly modifying them (eg `var.property = true`) will be an error.
- Similarly, the results of the useMemo/useCallback are treated as frozen (immutable) after the call.

These two rules match the behavior for other hooks: this means that developers will see similar behavior to swapping out `useMemo()` for a custom `useMyMemo()` wrapper/alias.

Additionally, as of #34503 the compiler uses information from the manual dependencies to know which variables are non-nullable. Even if a useMemo block conditionally accesses a nested property — `if (cond) { log(x.y.z) }` — where the compiler would not usually know that `x` is non-nullable, if the user specifies `x.y.z` as a manual dependency then the compiler knows that `x` and `x.y` are non-nullable and can infer a more precise dependency.

Finally, this mode also ensures that we always memoize function calls that return primitives. See #34343 for more details.

For now, I've explicitly opted out of this feature in all test fixtures where the behavior changed.