refactor(B8.3): move lambdas + captures to lower/closure.zig

Verbatim relocation of the 5-method closure cluster (lowerLambda,
bare-fn trampoline, closure-to-bare-fn adapter, capture collection, env
sizing) into src/ir/lower/closure.zig. 5 aliases on Lowering keep all
call sites unchanged. Method pub-flip: typeAlignBytes.

Resolves the B7.1 flag: CaptureInfo relocates from lower/call.zig to
lower/closure.zig (its domain home, next to collectCaptures); the
Lowering type alias is repointed so external references are unchanged.

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

src/ir/lower/call.zig

@@ -43,7 +43,6 @@ const Function = inst_mod.Function;
 const Module = mod_mod.Module;
 const Builder = mod_mod.Builder;
 
-
 const lower = @import("../lower.zig");
 const Lowering = lower.Lowering;
 const Scope = lower.Scope;
@@ -1206,13 +1205,6 @@ pub fn resolveBuiltin(name: []const u8) ?inst_mod.BuiltinId {
 
 // ── Lambda/closure ────────────────────────────────────────────
 
-pub const CaptureInfo = struct {
-    name: []const u8,
-    ty: TypeId,
-    ref: Ref, // alloca or value ref in the parent scope
-    is_alloca: bool,
-};
-
 /// Build `tp.name -> TypeId` bindings for a generic call.
 /// `args_ast` must be parallel to `fd.params`; for dot-calls the caller
 /// prepends the receiver's AST node so positions align with `fd.params[0] = self`.