fix(parser): parse braced defer { … } body as a statement block (issue 0065)

A braced `defer` body routed through `parseExpr` + a mandatory trailing
`;`, so it parsed the `{ … }` as a block-EXPRESSION whose statement loop
doesn't handle a destructure decl or a `catch`-statement — `defer { v, e
:= f(); … }` and `defer { x() catch e … }` failed with "expected ';'",
and even `defer { stmt; }` needed a spurious trailing semicolon.

Now the `kw_defer` arm parses a braced body with `parseBlock` (the same
path `onfail` uses), so every statement form works; the bare-expression
form (`defer expr;`) is unchanged. `in_defer_body` is still set before
parsing, so the cleanup-body control-flow bans (return/break/continue/
try/raise) and the E1.7 failable-absorption check still fire.

Resolves the `defer` manifestation of issue 0065 (the general
value-block-in-binding-position destructure remains open). Regression:
examples/1050-errors-defer-block-body.sx.

Gates: zig build, zig build test, run_examples.sh -> 341 passed, 0 failed.

examples/1050-errors-defer-block-body.sx

@@ -0,0 +1,27 @@
+// A braced `defer { … }` body parses as a full statement block (like `onfail`),
+// so it supports every statement form — a destructure decl, a `catch`-statement,
+// nested var decls — not just a single bare expression. Previously `defer { … }`
+// routed through the expression parser and rejected those with "expected ';'".
+//
+// Regression (issue 0065).
+
+#import "modules/std.sx";
+
+E :: error { Bad }
+
+probe   :: () -> (s32, !E) { return 21; }
+failing :: () -> !E { raise error.Bad; }
+
+run :: () {
+    defer {
+        v, e := probe();                              // destructure decl
+        if !e { print("defer: v={}\n", v); }          // value live under the guard
+        failing() catch x print("defer: caught\n");   // catch-statement absorbs
+    }
+    print("body\n");
+}
+
+main :: () -> s32 {
+    run();
+    return 0;
+}

examples/expected/1050-errors-defer-block-body.exit

@@ -0,0 +1 @@
+0

examples/expected/1050-errors-defer-block-body.stderr

@@ -0,0 +1 @@
+

examples/expected/1050-errors-defer-block-body.stdout

@@ -0,0 +1,3 @@
+body
+defer: v=21
+defer: caught