refactor(ir): extract ObjcLowering (ffi_objc.zig) for pure Obj-C decision helpers (A6.1 step 2)

Move the pure Obj-C decision helpers out of lower.zig into src/ir/ffi_objc.zig
behind an ObjcLowering *Lowering facade (Principle 5, like the A4/A5 resolvers).
Behavior-preserving relocation — the only non-self.l rewrites are facade
plumbing.

Moved verbatim (self. -> self.l. for Lowering members):
- deriveObjcSelector (selector derivation)
- objcTypeEncodingFromSignature + appendObjcEncoding + bailObjcEncoding +
  the ObjcEncodingStack type
- objcPropertyKind + the ObjcPropertyKind enum
- isObjcClassPointer
- objcDefinedStateStructType + objcStateAllocatorType

Emission-heavy code stays in lower.zig per PLAN A6.1 step 6: emitObjc* IMP
builders, lowerObjc*Call, registerObjc*, declareObjc*, the lookupObjc* property/
state lookups, and the Self-substitution resolvers.

- Call sites rerouted through a new objc() accessor: 15 in lower.zig, 1 in
  expr_typer.zig, 39 in lower.test.zig (the A6.1 scaffolding tests now drive the
  facade). No Lowering wrappers kept. Barrel-wired ffi_objc + ObjcLowering.
- No new visibility widening beyond sub-step 1's two pubs — the facade reads
  self.l.{alloc,module,program_index,diagnostics} (fields) + the already-pub
  resolveType. lower.zig -478 (->16615); ffi_objc.zig 428.
- Doc-only re-home: the property-IMP getter/setter comment was attached (a
  pre-existing artifact) to the moving ObjcPropertyKind enum, two decls away from
  its real subject emitObjcDefinedClassPropertyImps (which had no doc). Re-homed
  it there so the move neither orphans a `///` block (Zig errors on a dangling doc
  comment) nor misattributes it to ensureArcRuntimeDecls.

Gate: zig build, zig build test, bash tests/run_examples.sh -> 361/0
(48 13xx Obj-C examples + 4 Obj-C .ir snapshots green, no churn).

src/ir/ir.zig

@@ -14,6 +14,7 @@ pub const protocols = @import("protocols.zig");
 pub const conversions = @import("conversions.zig");
 pub const error_analysis = @import("error_analysis.zig");
 pub const error_flow = @import("error_flow.zig");
+pub const ffi_objc = @import("ffi_objc.zig");
 pub const semantic_diagnostics = @import("semantic_diagnostics.zig");
 
 pub const TypeId = types.TypeId;
@@ -54,6 +55,7 @@ pub const CoercionResolver = conversions.CoercionResolver;
 pub const CoercionPlan = conversions.CoercionResolver.CoercionPlan;
 pub const ErrorAnalysis = error_analysis.ErrorAnalysis;
 pub const ErrorFlow = error_flow.ErrorFlow;
+pub const ObjcLowering = ffi_objc.ObjcLowering;
 pub const ErrorFacts = error_analysis.ErrorFacts;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");