test/wcc/data: pin the drained divergences at runtime

The four graduated pin families byte-compared under test-data-byteid
but their original fixtures masked the miscompiles at runtime (lone
frame slot, dead code path, pointer accident). Add the discriminating
programs as run fixtures: adjacent-element clobber for the 2-byte
deref store, second-variant match dispatch for fn-type dedup, payload
integrity across the tagged widen of a padded struct, and base64
clear() actually zeroing its buffers. Corpus pins move to 1,229
fixtures / 137 run / 766 run-exit / 2,458 cells with the new identity
hash; the data-byteid floor moves to 915.

test/wcc/data/r660_i16_deref_store_adjacent/case.ww

@@ -0,0 +1,9 @@
+//ww:run-exit 42
+package main;
+fn setneg(p: *i16) void = { *p = -16i64: i16; };
+fn main() i32 = {
+	let a: [4]i16 = [7i16, 7i16, 7i16, 7i16];
+	setneg(&a[0]);
+	if (a[0]: i64 == -16i64 && a[1] == 7i16 && a[2] == 7i16 && a[3] == 7i16) { return 42; };
+	return 1;
+};