[STUBGEN][RUST] Generate complete object mirrors and allocators

[Seven-Streams]

Summary

Attach the layout classifier (#730) to the Rust backend. A type whose layout the registry proves is now bound complete: a #[repr(C)] struct with every physical field at its reflected offset and width, a const size/alignment assertion, and a lossless allocator (<Leaf>Obj::new crate-private, <Leaf>::new public). Everything else keeps the opaque form of #738. Three new directives: opaque vetoes a reproducible layout, upcast adds a hand-written typed view, custom-new renames the generated allocator to from_complete_fields so a hand-written new can own the name.

Motivation

This is what tvm-rust-ext's STUBGEN_FEEDBACK.md asks for: ordinary data nodes allocated in Rust from their complete fields, polymorphic and unreflected-byte types kept opaque. Generating ir. / tirx. / arith. / target. from tvm-rust-ext's libtvm_compiler.so gives 131 types (98 complete, 33 opaque) that compile against the crate without edits; the differences from the hand-written bindings are exactly what the directives cover.

Changes

• stub/layout.py: classify(..., unmirrored=...) and the no-mirror reason, so types under a builtin parent (ffi.Enum, ...) stay opaque: their base is only a header-only stand-in.
• rust_generator/codegen.py: classify each object with its ancestors; mirror fields (scalars by reflected width, Optional<T> as Option<T> or tvm_ffi::Optional<T> by payload, directive widths checked against the field size); render the complete struct and its allocators; upcast and custom-new handling.
• rust_generator/directives.py, consts.py: the three directives and the width tables.
• rust_generator/utils.py: a reflected field named base or data is spelled base_ / data_, since those are the generated struct's own members (TVM has tirx.Ramp.base, tirx.DeclBuffer.data).
• examples/rust_stubgen/: IntPair is now a plain data object, allocated from Rust with the generated IntPair::new and read back by C++.

Testing

test_stubgen_rust.py (36 cases), test_stubgen.py, test_stub_layout.py: 121 passed; ruff clean. The example regenerates an identical mod.rs and cargo run prints a=1 b=2 kind=PairKind(1) and sum=3.