lib: expectabort rows for the restored preconditions; retire 989_libprecond_abort.c

The three must-abort runs (u32n/u64n zero, decodedsize unaligned) move
into the owning lib suites as test.expectabort rows; the five
normal-return controls already live in the r989_libprecond_* fixtures.
wwstage parity rides the lib-wide cs==ww byte identity gate.

lib/encoding/base64/base64_test.ww

@@ -16,6 +16,7 @@ import errors;
 import io;
 import memio;
 import strings;
+import test;
 
 
 fn streq(a: str, b: str) bool = {
@@ -162,6 +163,15 @@ fn inval_check(enc: *base64.encoding, encoded: str) void = {
 	assert(!(base64.decodedsize(164) != 123));
 };
 
+// A non-multiple-of-4 input is out of domain and must abort loudly,
+// never return garbage. Pins the restored Hare precondition
+// (ref/hare/encoding/base64/base64.ha:597, assert(sz % 4 == 0)).
+
+@test fn decodedsize_unaligned_aborts() void = {
+	test.expectabort();
+	base64.decodedsize(5i32);
+};
+
 // ---- round-trip every byte value 0..255 (std + url) ----
 
 @test fn roundtrip_all_bytes() void = {