w6c: remap legacy-union error tags in wwstage ? propagation

The #173 remap loop in cgtryprop gated on the tparam's explicit `!`
flag only; a LEGACY union (no marks anywhere) classifies error
variants POSITIONALLY (index 0 is success, cstage cg_variant_is_error
cgen.c:905) — so wwstage emitted no remap and propagated the callee's
raw tag into a differently-ordered caller union, a silent wrong arm
(cstage exit 7, wwstage 9 on the banked r700 row). Add the
variantiserror twin and gate the loop on it; refresh the drifted
cstage cite.

Graduates the held e2e row into the corpus (pin 1727/1156/3454) and
adds legacy-mode rows to test/lang/tryprop_tag_remap_test.ww:
reversed order both directions, slot-0 remap, str payload integrity,
a two-hop chain whose pre-fix runtime pass was double-miss tag
cancellation (the lang byteid leg pins the emitted remap blocks),
and a same-order zero-emission control.

test/wcc/data/r700_try_rev_errs/case.ww

@@ -0,0 +1,21 @@
+//ww:run-exit 7
+// Migrated from 700_e2e row 69.
+package main;
+fn inner(n: i32) (i32 | str | bool) = {
+  if (n == 0) { return "z"; };
+  if (n < 0)  { return false; };
+  return n;
+};
+fn outer(n: i32) (i64 | bool | str) = {
+  let v: i32 = inner(n)?;
+  return v: i64 + 1000;
+};
+fn main() i32 = {
+  let r: (i64 | bool | str) = outer(-1);
+  match (r) {
+    case let n: i64  => return n: i32;
+    case let b: bool => { if (!b) { return 7; }; return 8; };
+    case let s: str  => return 9;
+  };
+  return 0;
+};