cleanup(B-review): fix stale section banners, merge alias block, prune imports

Review follow-up to the ARCH-B split (comment/import hygiene only, no
code changes):

- Section banners that travelled to the wrong file with the B1-B8 cuts
  are reworded to describe the section that actually follows (e.g.
  stmt.zig's trailing "Expression lowering", expr.zig's "Control flow"
  before lowerChainedComparison) or deleted where nothing follows
  (4 trailing-at-EOF banners). ffi.zig's facade note no longer claims
  the IMP builders "stay here" (they live in lower/objc_class.zig);
  protocol.zig's namespace-lookup banner now points at
  pack.zig:resolvePackProjection for the orchestrator.
- lower.zig's two lower/expr.zig alias blocks (B8.1 + B8.2 appends)
  merged into one.
- 448 unused header decls pruned from the 15 lower/*.zig files (each
  had inherited lower.zig's full import block; pruned to fixpoint so
  cascading type-extraction consts went too).

Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.

src/ir/lower/expr.zig

@@ -1,5 +1,4 @@
 const std = @import("std");
-const Allocator = std.mem.Allocator;
 const ast = @import("../../ast.zig");
 const Node = ast.Node;
 const types = @import("../types.zig");
@@ -7,41 +6,15 @@ const inst_mod = @import("../inst.zig");
 const mod_mod = @import("../module.zig");
 const type_bridge = @import("../type_bridge.zig");
 const unescape = @import("../../unescape.zig");
-const parser_mod = @import("../../parser.zig");
-const interp_mod = @import("../interp.zig");
 const errors = @import("../../errors.zig");
-const jni_descriptor = @import("../jni_descriptor.zig");
-const program_index_mod = @import("../program_index.zig");
-const resolver_mod = @import("../resolver.zig");
-const imports_mod = @import("../../imports.zig");
-const ProgramIndex = program_index_mod.ProgramIndex;
-const GlobalInfo = program_index_mod.GlobalInfo;
-const StructTemplate = program_index_mod.StructTemplate;
-const TemplateParam = program_index_mod.TemplateParam;
-const ProtocolDeclInfo = program_index_mod.ProtocolDeclInfo;
-const ProtocolMethodInfo = program_index_mod.ProtocolMethodInfo;
-const ModuleConstInfo = program_index_mod.ModuleConstInfo;
 const TypeResolver = @import("../type_resolver.zig").TypeResolver;
-const ResolveEnv = @import("../type_resolver.zig").ResolveEnv;
-const PackResolver = @import("../packs.zig").PackResolver;
-const ExprTyper = @import("../expr_typer.zig").ExprTyper;
-const CallResolver = @import("../calls.zig").CallResolver;
-const GenericResolver = @import("../generics.zig").GenericResolver;
-const ProtocolResolver = @import("../protocols.zig").ProtocolResolver;
-const CoercionResolver = @import("../conversions.zig").CoercionResolver;
-const ErrorAnalysis = @import("../error_analysis.zig").ErrorAnalysis;
-const ErrorFlow = @import("../error_flow.zig").ErrorFlow;
-const ObjcLowering = @import("../ffi_objc.zig").ObjcLowering;
-const semantic_diagnostics = @import("../semantic_diagnostics.zig");
 
 const TypeId = types.TypeId;
 const StringId = types.StringId;
 const Ref = inst_mod.Ref;
-const BlockId = inst_mod.BlockId;
 const FuncId = inst_mod.FuncId;
 const Function = inst_mod.Function;
 const Module = mod_mod.Module;
-const Builder = mod_mod.Builder;
 
 
 const lower = @import("../lower.zig");
@@ -1424,7 +1397,7 @@ pub fn resolveOptionalInner(self: *Lowering, ty: TypeId) TypeId {
     return .unresolved;
 }
 
-// ── FFI intrinsics (#objc_call / #jni_call / #jni_static_call) ─
+// ── Core expression dispatch ───────────────────────────────────
 
 pub fn lowerExpr(self: *Lowering, node: *const Node) Ref {
     // Stamp this node's source span onto the instructions it emits (ERR
@@ -2344,7 +2317,7 @@ pub fn lowerTupleMembership(self: *Lowering, value: Ref, tuple: Ref, tuple_info:
     return result;
 }
 
-// ── Control flow ────────────────────────────────────────────────
+// ── Chained comparison ──────────────────────────────────────────
 
 pub fn lowerChainedComparison(self: *Lowering, cc: *const ast.ChainedComparison) Ref {
     // a < b < c → (a < b) and (b < c)
@@ -2381,5 +2354,3 @@ pub fn emitCmp(self: *Lowering, lhs: Ref, rhs: Ref, op: ast.BinaryOp.Op) Ref {
         else => self.builder.constBool(false),
     };
 }
-
-// ── Defer/Push/MultiAssign ──────────────────────────────────────