Add dual_stack_differential to e2e_test. It starts two rustfs processes
from the same binary, stack A on RUSTFS_S3_STACK=legacy and stack B on
RUSTFS_E2E_STACK_B (default legacy until rustfs/backlog#2734 T2.5),
sends both the same aws-sdk-s3 script covering the 73 methods of
impl S3 for FS (a normal request and a 4xx negative each), and compares
status, error code, an allowlist of response headers and a few typed
body fields. Error message text is never compared. Each run writes
<target>/dual-stack/<run>.json with compared_ops and the (op, field,
a, b) differences; any difference fails the test.
differential_detects_an_injected_difference starts B with
RUSTFS_REGION=eu-west-1 and requires GetBucketLocation to be reported.
The comparator is pure and unit-tested without a server.
Wire the module into the e2e-smoke filter and update the e2e-smoke and
e2e-full selection digests and the e2e_test test-count row.
Refs rustfs/backlog#2740
Chris
f600af4704