refactor(ir): extract ProgramIndex, move low-fanout decl facts (A1.1a)

Architecture phase A1.1a. Introduce src/ir/program_index.zig as the single
storage owner for declaration-name / import / visibility facts, and move the
three low-fanout maps out of the Lowering state bag:

- import_flags     (owned by ProgramIndex)
- module_scopes    (borrowed pointer into a core.zig-owned map)
- import_graph     (borrowed pointer into a core.zig-owned map)

Lowering embeds one ProgramIndex by value and reaches every moved fact through
self.program_index.<field>; later phases hand collaborator modules a
*ProgramIndex instead of *Lowering. 8 call sites in lower.zig + 2 setters in
core.zig repointed. No duplicate storage, no fallback path; zig build enforces
no missed reference.

Mutation-heavy registration (registerStructDecl etc.) stays in Lowering and
now writes import_flags through the index. High-fanout maps are deferred to
A1.1b.

Adds src/ir/program_index.test.zig (init-empty, import_flags round-trip,
borrowed-view ownership) wired into the ir.zig barrel.

Behavior-preserving: zig build, zig build test, and bash tests/run_examples.sh
(350 passed, 0 failed) all green.

src/ir/ir.zig

@@ -4,6 +4,7 @@ pub const module = @import("module.zig");
 pub const print = @import("print.zig");
 pub const interp = @import("interp.zig");
 pub const lower = @import("lower.zig");
+pub const program_index = @import("program_index.zig");
 
 pub const TypeId = types.TypeId;
 pub const TypeInfo = types.TypeInfo;
@@ -30,6 +31,7 @@ pub const printModule = print.printModule;
 pub const Interpreter = interp.Interpreter;
 pub const Value = interp.Value;
 pub const Lowering = lower.Lowering;
+pub const ProgramIndex = program_index.ProgramIndex;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");
 pub const emit_llvm = @import("emit_llvm.zig");
@@ -48,6 +50,7 @@ pub const module_tests = @import("module.test.zig");
 pub const print_tests = @import("print.test.zig");
 pub const interp_tests = @import("interp.test.zig");
 pub const lower_tests = @import("lower.test.zig");
+pub const program_index_tests = @import("program_index.test.zig");
 pub const type_bridge_tests = @import("type_bridge.test.zig");
 pub const emit_llvm_tests = @import("emit_llvm.test.zig");
 pub const jni_descriptor_tests = @import("jni_descriptor.test.zig");