fix(rt-data): handle singleton-array projection traversal

[Stevengre]

Summary

This adds singleton-array runtime projection support in rt/data.md and a minimal prove-rs reproducer for pointer-cast writes through *mut [u8; 1].

Context

The runtime already had type/projection plumbing for projectionElemSingletonArray, but write paths still had no #traverseProjection rule for that projection. When a pointer is cast to a singleton array pointer and then written through, execution reaches #traverseProjection(..., projectionElemSingletonArray ...) and stops.

Red vs Green

Red:

• singleton-array-pointer-cast-fail.rs got stuck at #traverseProjection(..., projectionElemSingletonArray .ProjectionElems, .Contexts).
• The same family shows up downstream in close_account_multisig through std::ptr::write_volatile::<[u8; 32]>.

Green:

• #traverseProjection wraps the value in a singleton array context.
• #buildUpdate unwraps that singleton array on the way back out.
• #projectionsFor reconstructs the projection path correctly, so the reproducer now runs as the passing fixture singleton-array-pointer-cast.rs and reaches #EndProgram instead of stopping at projection traversal.

References

• Rust Reference: type cast expressions
  • https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions
• Rust standard library: std::ptr::write
  • https://doc.rust-lang.org/std/ptr/fn.write.html

[Stevengre]

Closing: reproducer passes on current master without any rule changes. The consP cancellation rules already handle SingletonArray + ConstantIndex(0) during projection construction. For pre-built projection chains (like write_volatile), PR #988 covers the fix.