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.zig

@@ -500,7 +500,7 @@ pub const Lowering = struct {
         };
     }
 
-    // ── Public entry point ──────────────────────────────────────────
+    // ── Layout delegators ───────────────────────────────────────────
 
     /// Byte size of an IR type matching LLVM's type layout.
     pub fn typeSizeBytes(self: *Lowering, ty: TypeId) usize {
@@ -516,7 +516,7 @@ pub const Lowering = struct {
         return .void;
     }
 
-    // ── Chained comparison ──────────────────────────────────────────
+    // ── Type-resolution delegators ──────────────────────────────────
 
     pub fn resolveReturnType(self: *Lowering, fd: *const ast.FnDecl) TypeId {
         if (fd.return_type) |rt| {
@@ -1645,8 +1645,6 @@ pub const Lowering = struct {
     pub const lowerForceUnwrap = lower_expr.lowerForceUnwrap;
     pub const lowerNullCoalesce = lower_expr.lowerNullCoalesce;
     pub const resolveOptionalInner = lower_expr.resolveOptionalInner;
-
-    // --- moved to lower/expr.zig (lower_expr) ---
     pub const lowerExpr = lower_expr.lowerExpr;
     pub const refCapturePointee = lower_expr.refCapturePointee;
     pub const lowerBinaryOp = lower_expr.lowerBinaryOp;