rdl writer: propagate SupportedArchitecture to un-nested inner types

[Copilot]

When the RDL writer un-nests anonymous inner types from arch-restricted structs/unions, the synthesised helper types (e.g. SLIST_HEADER_0) were emitted without any SupportedArchitecture attribute, incorrectly making them appear available on all architectures.

Changes

• collect_nested — adds parent_arches: i32 parameter; computes effective_arches = parent_arches | nested.arches() per inner type so unrestricted inner types inherit the parent's constraint. Propagated recursively for deeply-nested cases. Now also emits any other custom attributes on inner types (previously none were written).
• write_arch_attr — new helper mapping the arch bitmask (bit 0 = X86, bit 1 = X64, bit 2 = Arm64) to #[Windows::Win32::Foundation::Metadata::SupportedArchitecture(...)] tokens; returns empty stream for zero or unrecognised bits.
• write_struct_items — seeds collect_nested with item.arches() from the outermost type.
• tests/nested-arches — regression test filtering Windows.Win32.System.Kernel; the committed nested-arches-out.rdl shows SLIST_HEADER_0/SLIST_HEADER_1 each carrying the correct per-arch attribute.

// Before fix — un-nested inner types had no arch attribute:
struct SLIST_HEADER_0 { Alignment: u64, Region: u64, }

// After fix — arch inherited from parent SLIST_HEADER:
#[Windows::Win32::Foundation::Metadata::SupportedArchitecture(Arm64)]
struct SLIST_HEADER_0 { Alignment: u64, Region: u64, }
#[Windows::Win32::Foundation::Metadata::SupportedArchitecture(X64)]
struct SLIST_HEADER_0 { Alignment: u64, Region: u64, }
#[Windows::Win32::Foundation::Metadata::SupportedArchitecture(X86)]
struct SLIST_HEADER_0 { Next: SINGLE_LIST_ENTRY, Depth: u16, CpuId: u16, }