Claude review: rust: ptr: add projection infrastructure

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

Patch Review

This is the most complex patch and forms the core of the series. The design is solid.

**Deref guard**: The `ProjectField` trait's dual implementation (with `const DEREF: bool`) to create ambiguity when a type implements `Deref` is clever and prevents a real soundness issue.

**Alignment check**: The `if false { let _ = &(*ptr).$field; }` trick to reject `#[repr(packed)]` field projections at compile time is elegant — the compiler checks alignment even in dead code.

**MaybeUninit approach** in `ProjectField::proj`:

```rust
let mut place = MaybeUninit::uninit();
let place_base = place.as_mut_ptr();
let field = f(place_base);
let offset = unsafe { field.byte_offset_from(place_base) };
base.wrapping_byte_offset(offset).cast()
```

The comment notes this should work even for very large `T` as the memory is never used and should be optimized away. This is the same approach as the `memoffset` crate. It's sound but relies on the optimizer — for extremely large types, debug builds could theoretically cause stack issues here too. In practice this is fine.

**`Makefile.build` changes**: Adding `slice_ptr_len` and `strict_provenance` to `rust_allowed_features` is correct and the comments correctly document the stabilization versions.

No blocking issues.

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Generated by Claude Code Patch Reviewer