test: add back previous generated xdr into test fixture for roundtrip testing

Author: Ryang-21

test/fixtures/legacy-xdr/README.md

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+# Legacy XDR fixtures
+
+These are the `@stellar/js-xdr`-backed generated XDR classes as they existed
+at master (commit `6ba32bc5^`), copied here verbatim. They are **not** shipped
+as part of the library — `src/` does not import from this directory.
+
+## Role
+
+Source of truth for byte-compatibility testing. Round-trip tests in
+`test/unit/base/xdr/` use this layer as an oracle: encode the same logical
+value through both the new class-based runtime (`src/xdr/`) and this legacy
+layer, then assert byte equality. If the bytes diverge, the new layer has a
+bug.
+
+## Files
+
+- `curr_generated.{js,d.ts}` — generated XDR primitives for the curr
+  protocol, depend on `@stellar/js-xdr` (still in `dependencies`).
+- `curr.d.ts` — high-level type wrapper around `curr_generated`.
+- `next_*` — same, for the next protocol.
+
+The only edit from master is fixing the `OperationRecord` import path in
+`curr.d.ts` / `next.d.ts` (was `../operations/types.js`, now points at
+`src/base/operations/types.js` via three-levels-up).
+
+## When to delete
+
+Once the class-based runtime has been in production for a couple releases
+and we're confident in byte-for-byte equivalence, this directory can be
+removed. The round-trip tests stay relevant via the new-XDR side; the
+legacy oracle is a transitional confidence-builder.