Add payment stream example - #303

- Bump XDR_COMMIT to stellar-xdr@68fa1ac (post-stellar#303 which ungated CAP-0071).
- Clear XDR_FEATURES default so CAP-0083 #ifdef blocks are stripped during
  preprocess. CAP-0071 is ungated and needs no feature flag.
- Regenerate gxdr/, xdr/xdr_generated.go, xdr/xdr_views_generated.go.
- randxdr: add IsDeepNestedDelegates preset to cap recursion through CAP-71
  SorobanDelegateSignature.nestedDelegates, mirroring the existing
  IsDeepAuthorizedInvocationTree handling for subInvocations. Without it
  TestView_RandXDR_RawRoundTrip stack-overflows on unbounded random trees.

* xdr: regenerate for Protocol 27 CAP-0071 + CAP-0083

Bump XDR to stellar-xdr@5187e69 (CAP-0071 address-bound/delegated Soroban
credentials + CAP-0083 STELLAR_VALUE_EMPTY_TX_SET). goxdr and Ruby xdrgen
cannot parse #ifdef, so CAP_0071/CAP_0083 gates are resolved with
'stellar-xdr xfile preprocess' (rs-stellar-xdr #503) before codegen, driven
by XDR_FEATURES. Regenerated xdr/, gxdr/, and xdr/xdr_views_generated.go.

* xdr: drop CAP-0083 from regen; only CAP-0071 ships in p27

- Bump XDR_COMMIT to stellar-xdr@68fa1ac (post-#303 which ungated CAP-0071).
- Clear XDR_FEATURES default so CAP-0083 #ifdef blocks are stripped during
  preprocess. CAP-0071 is ungated and needs no feature flag.
- Regenerate gxdr/, xdr/xdr_generated.go, xdr/xdr_views_generated.go.
- randxdr: add IsDeepNestedDelegates preset to cap recursion through CAP-71
  SorobanDelegateSignature.nestedDelegates, mirroring the existing
  IsDeepAuthorizedInvocationTree handling for subInvocations. Without it
  TestView_RandXDR_RawRoundTrip stack-overflows on unbounded random trees.

* go.mod: bump go-xdr to 0bf8f49 (raise DecodeDefaultMaxDepth to 1500)

Pulls in stellar/go-xdr#32 which lifts the default XDR decode depth from
250 → 1500 and adds the DecodeUnlimitedDepth sentinel. Necessary so the
CAP-71 SorobanDelegateSignature delegate chain (which the protocol allows
to nest arbitrarily, with only Soroban's runtime trap policing depth) can
be decoded by stellar-core LCM consumers without stalling.

* go.sum: drop stale go-xdr@a87d4d0 entry (go mod tidy)

The previous commit bumped go-xdr to 0bf8f49 in go.mod, but go.sum still
carried the old a87d4d0789c3 lines. CI's gomod.sh runs `go mod tidy` and
fails the build on the residual diff.

* ci: bump go-test timeout to 20m

The default per-test-binary timeout is 10m, which is borderline for
this repo's race-cover suite (xdr/ alone takes ~230s with -race
locally, longer on slower CI runners). CAP-71's added generated
XDR pushed the run past 10m, killing the test job with SIGTERM
before xdr/ and txnbuild/ finished.

20m gives generous headroom and matches the conservative timeout
other Stellar Go repos use for race+cover runs.

* txnbuild: cap CAP-71 nestedDelegates recursion in TestOperationCoverage

TestOperationCoverage generates 10,000 random gxdr.Operation values
which now include InvokeHostFunctionOp.auth[].credentials carrying
the new SorobanAddressCredentialsWithDelegates with the recursive
nestedDelegates field. Without bounding, the random generator
recurses unboundedly through SorobanDelegateSignature.nestedDelegates
and the test hangs past go test's default 10-minute per-binary
timeout, killing the CI test matrix.

Add IsDeepNestedDelegates preset (already defined in
randxdr/presets.go for LedgerCloseMetaPresets) — caps the tree height
at 2. With the preset, the test runs in ~8s under -race instead of
timing out.

Pairs with the -timeout=20m bump on the test workflow (which is now
just headroom — the real fix is this preset).