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

首先定位用于 list[i]、tuple[i]、str[i] 和 list[i] = value 的专用 int 下标快速路径。检查它们的 guard 如何表达 compact-int 前置条件，以及负索引如何到达 fallback path。完成的标准是：guard 要求非负 compact int，移除 specialized body 中冗余的检查，并且负索引的行为保持不变。