issues 0033 + 0034: rename repros to focused regression tests

Both repros emit their target diagnostics cleanly today (verified
2026-05-28 against HEAD):

- `issue-0033` → "no visible xx conversion from 's64' to 'Wrap'
  — impl exists in another module but is not imported". Catches
  the case where an `impl Into(X) for Y` is registered globally
  via one module's import chain but is NOT transitively imported
  by the file containing the `xx` site.
- `issue-0034` → "duplicate xx conversion from 's64' to 'Wrap':
  impls in <a> and <b>". Catches two impls covering the same
  (Source, Target) pair both reachable from a single `xx` site.

Renamed to focused feature names:

- `issue-0033*` → `179-impl-visibility*` (4 files: main + impl +
  types + user).
- `issue-0034*` → `180-impl-duplicate*` (4 files: main + impl-a +
  impl-b + types).

Path references inside the files updated. Comment headers tightened
to feature-focused (drop issue-NNNN provenance — that's in git
history now). Expected `.txt` / `.exit` files captured against the
full diagnostic text and exit code 1.

The `issue-*` namespace in `examples/` now shrinks to the literal
list of UNRESOLVED bug repros. 218/218.

examples/180-impl-duplicate.sx

@@ -0,0 +1,23 @@
+// Duplicate impl detection — two impls for the same (Source, Target)
+// pair are both visible from the same `xx` site (because both their
+// defining modules are transitively imported). The compiler must
+// emit a "duplicate xx conversion" diagnostic naming both modules,
+// not silently pick one or the other.
+//
+// Setup:
+//   - 180-impl-duplicate-impl-a.sx: `impl Into(Wrap) for s64` (mul by 10).
+//   - 180-impl-duplicate-impl-b.sx: `impl Into(Wrap) for s64` (add 100).
+//   - Main imports both; the `xx 7 : Wrap` site must error.
+//
+// Expected exit = 1, expected output = the focused diagnostic naming
+// both impl modules.
+
+#import "modules/std.sx";
+#import "./180-impl-duplicate-impl-a.sx";
+#import "./180-impl-duplicate-impl-b.sx";
+
+main :: () -> s32 {
+  w : Wrap = xx 7;
+  print("w.v = {}\n", w.v);
+  0;
+}