feat: implement arch-specific types support (issue #4299)

[Copilot]

• Part 1: Add target() method to Clang builder
• Part 2: Add #[arch(...)] shorthand in writer/reader
  • write_arch_attr() moved to writer/mod.rs, emits #[arch(...)]
  • Arch attribute for struct/union/fn writers
  • read_arch(), parse_arch_bitmask(), arch_name_to_bits() in reader
  • emit_arch_attribute() in attribute_ref.rs
  • encode_attrs skips arch globally
  • encode_struct, encode_union, encode_fn handle #[arch(...)]
  • Updated golden files (roundtrip/arches.rdl, nested-arches.rdl)
  • Negative tests for invalid arch names
• Part 3: Multi-arch merge in windows-metadata Merger
  • arch_input(path, arch_bits) method
  • Arch-aware merge() logic
  • write_type with arch_override
  • write_attributes_with_arch() helper
  • test_arch test + golden files
• cargo fmt

[kennykerr]

@copilot fmt

[Copilot]

@copilot fmt

Done — formatted in d72d4e3.