transpiler: support opt-in scalar value initialization

[shuaimu]

Summary

• add a fail-closed #[cfg_attr(any(), cpp_value_init)] marker for named scalar fields on ordinary Rust structs
• emit a C++ default member initializer (field{}) only for annotated bool/integer fields, leaving unmarked fields unchanged
• validate the exact inert marker spelling, placement, cardinality, and supported types before code generation
• wire validation through both ordinary and pre-lowered transpilation paths

Why

Raft message structs need selected scalar fields to retain historical C++ plain-default-construction semantics while remaining valid Rust source. The inert marker gives code generation that narrow ABI/behavior hint without adding constructors or changing aggregate layout or positional initialization.

Validation

• unit and golden coverage accepts bool and signed/unsigned integer primitive fields and rejects malformed spellings, duplicates, unsupported types, and invalid placements
• compile-and-run coverage checks generated spelling, scalar zeroing, aggregate/standard-layout/trivially-copyable traits, field offsets, size/alignment, and positional initialization