Claude review: rust: ptr: use `match` instead of `unwrap_or_else` for `build_index`

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Replaces `unwrap_or_else(|| build_error!())` with a `match` to avoid potential inlining issues:

```rust
-        Self::get(self, slice).unwrap_or_else(|| build_error!())
+        match Self::get(self, slice) {
+            Some(v) => v,
+            None => build_error!(),
+        }
```

This is a good defensive change. `build_error!()` is a compile-time construct and relying on the closure being inlined (so the compiler can see the `build_error!` call) is fragile. The `match` makes this unconditional. Suggested by Alice Ryhl with a link to discussion. No issues.

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