Specialized subscript guards should require non-negative compact ints

The specialized int subscript fast paths currently assume a non-negative compact int, but that precondition is not expressed directly in the guard.

Make the guard explicit for the affected cases:

• list[i]
• tuple[i]
• str[i]
• list[i] = value

This aligns the guard with the existing specialization contract and removes redundant checks from the specialized bodies, without changing the fallback path for negative indices.

Linked PRs

• gh-153487
• gh-153523

首先定位用於 list[i]、tuple[i]、str[i] 和 list[i] = value 的專用 int 下標快速路徑。檢查它們的 guard 如何表達 compact-int 前置條件，以及負索引如何到達 fallback path。完成的標準是：guard 要求非負 compact int，移除 specialized body 中多餘的檢查，並且負索引的行為保持不變。