refactor(ir): move declaration maps into ProgramIndex (A1.1b)

Architecture phase A1.1b — mechanical storage relocation. Move the 9
declaration-fact maps out of the Lowering state bag into ProgramIndex:

  high-fanout:   fn_ast_map, foreign_class_map, global_names, type_alias_map
  medium-fanout: struct_template_map, protocol_decl_map, protocol_ast_map,
                 module_const_map, ufcs_alias_map

168 self.<map> sites in lower.zig repointed to self.program_index.<map>;
external readers repointed too (core.zig foreign_class_map iteration;
lower.test.zig fn_ast_map / foreign_class_map). No duplicate storage, no
fallback path; zig build enforces no missed reference.

The four maps whose value types were Lowering-private pull those types into
program_index.zig as pub (GlobalInfo, StructTemplate + TemplateParam,
ProtocolDeclInfo + ProtocolMethodInfo, ModuleConstInfo); lower.zig aliases
them at file scope so call sites are unchanged.

Behavior is preserved exactly:
- per-map allocator unchanged — import_flags/fn_ast_map/global_names use the
  lowering allocator (ProgramIndex.init), the other 7 keep their page_allocator
  inline defaults;
- ProgramIndex.deinit frees only the 10 owned maps, never the borrowed
  module_scopes / import_graph;
- TypeTable.aliases still borrows &self.program_index.type_alias_map, loaned at
  lowerRoot with the same late-binding lifetime.

Extends program_index.test.zig with declaration-map round-trips (fn AST, type
alias, global, module const, foreign class, protocol decl/AST, struct template,
ufcs alias).

Registration logic (registerStructDecl / registerProtocolDecl /
registerForeignClassDecl, ...) stays in Lowering, writing through the index.

Gate green: zig build, zig build test, bash tests/run_examples.sh
(350 passed, 0 failed). lower.zig 19433 -> 19393 lines.

src/ir/lower.test.zig

@@ -479,7 +479,7 @@ test "lower: objcTypeEncodingFromSignature emits @ for Obj-C class pointers" {
         .is_foreign = true,
         .is_main = false,
     };
-    try lowering.foreign_class_map.put("NSString", &ns_fcd);
+    try lowering.program_index.foreign_class_map.put("NSString", &ns_fcd);
 
     // Return *NSString, no args: "@@:"
     const e1 = try lowering.objcTypeEncodingFromSignature(ns_ptr, &.{}, null);
@@ -510,7 +510,7 @@ test "lower: objcTypeEncodingFromSignature unwraps optional to wire type" {
         .is_foreign = true,
         .is_main = false,
     };
-    try lowering.foreign_class_map.put("NSString", &ns_fcd);
+    try lowering.program_index.foreign_class_map.put("NSString", &ns_fcd);
 
     // `?s64 -> ?*NSString` collapses to `q -> @` at the Obj-C boundary.
     const opt_s64 = module.types.optionalOf(.s64);
@@ -750,8 +750,8 @@ test "E1.4b converge inferred error sets: empty -> warning, raising -> converged
     r_body.* = .{ .span = .{ .start = 0, .end = 0 }, .data = .{ .block = .{ .stmts = r_stmts } } };
     const raiser_fd = ast.FnDecl{ .name = "raiser", .params = &.{}, .return_type = r_rt, .body = r_body };
 
-    lowering.fn_ast_map.put("stub", &stub_fd) catch unreachable;
-    lowering.fn_ast_map.put("raiser", &raiser_fd) catch unreachable;
+    lowering.program_index.fn_ast_map.put("stub", &stub_fd) catch unreachable;
+    lowering.program_index.fn_ast_map.put("raiser", &raiser_fd) catch unreachable;
 
     lowering.convergeInferredErrorSets();