Add opt-in Objective-C binding generation

[tannergooding]

Opt-in Objective-C binding generation, behind --generate-objective-c-bindings. This is the first phased slice of #254 and mirrors how we already emit COM: expose the raw C ABI and a friendly wrapper on the same type, keeping the layout faithful.

The parse/interop half of #254 has been in place for years (ClangSharp self-hosts the full Clang AST, and clangsharp_Cursor_getSelector already exists), so this PR is emitter-only.

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Model

An @protocol/@interface/category maps to a public unsafe partial struct whose only field is void* isa — a byte-for-byte struct objc_object. The object pointer is this; isa is the identity/class hook. Unlike COM's lpVtbl there is no fixed dispatch table, because Objective-C dispatch is dynamic through objc_msgSend. Modern non-fragile ivars are exactly why the struct is opaque rather than a fixed-layout record.

Each in-subset method/property emits: a cached SEL in a nested public static partial class Selectors, and a friendly member that casts the single raw objc_msgSend entry point to the call site's exact delegate* unmanaged<...> signature and invokes it with the implicit (self, _cmd) leading args. Instance members recover self via Unsafe.AsPointer(ref this); class (+) members are static and dispatch on objc_getClass(...). A small inline ObjectiveC/SEL runtime surface is emitted once per output.

ABI

Targets arm64 only, and says so. On Apple Silicon there is no objc_msgSend_stret/_fpret split — struct and FP returns/args all go through the single objc_msgSend under AAPCS64, which is what the JIT follows for the generated signature. By-value aggregates are therefore emitted through plain objc_msgSend with the real struct type, each flagged with a one-line note that x86-64 _stret is intentionally unsupported (Apple dropped x86-64 macOS). This sidesteps per-triple variant selection entirely.

What's included

• Opt-in flag + decl dispatch for ObjCProtocol, ObjCInterface, ObjCCategory (goto default when off).
• @protocol → opaque struct + isa + Selectors + friendly instance methods/properties.
• @interface → same, plus the backing Class object and class (+) members.
• @required/@optional surfaced, with a respondsToSelector:-guard note on optional members.
• Inheritance + adopted protocols recorded in [NativeTypeName]; members are not flattened (dynamic dispatch + identical layout means a pointer cast to the superclass just works).
• ObjCObjectPointerType mapping (id → void*, NSObject * → NSObject*).
• By-value aggregate return/param support (arm64, flagged).
• Categories reopen the class's partial struct and partial Selectors, adding only new members.
• Out-of-subset constructs are diagnosed and skipped rather than mis-emitted: variadic methods, and class members on a @protocol (no backing class).
• 8 golden tests (Windows + Unix variants where relevant); full suite green, 0 warnings.

Non-goals (intentional)

• Variadic methods — a C ... can't be a fixed function-pointer signature.
• @implementation — definition-side; adds no callable surface over the @interface.
• Foundation/SDK base types — users point the generator at the SDK headers, same as any other platform SDK.
• x86-64 macOS — dropped by Apple; the _stret/_fpret variants are not supported.
• ARC / retain-release — no C# equivalent; ownership is the caller's, as with any unsafe interop.

Before this is ready to merge

• API review for the new config option and emitted public surface.
• A decision on whether the inline ObjectiveC/SEL runtime surface should stay generated, move to a small hand-written support library, or be left to the consumer.
• On-device arm64 validation — the golden tests prove shape, not runtime correctness.

Contributes to #254.