Real ShardRuntimeServicer process bound to a real localhost socket, driven
by a generated ShardRuntimeStub over grpc.insecure_channel from a separately
spawned subprocess. Proves direct-hop and opaque-relay (exact captured
request bytes re-sent, no reinterpretation) produce byte-identical server
responses, cross-checked against an independent server-side wire capture.
Fails closed on the required negative paths: stale route epoch, expired
deadline, malformed/non-tiling fragments, checksum failure, exhausted
flow-control credit (with in-band top-up), duplicate idempotency steps
(acked, not re-applied), and cancel — both in-band CancelSignal (single
work item vs whole session) and the out-of-band unary Cancel RPC, including
a Cancel that races ahead of SessionOpen.
Supersedes the earlier in-memory fake-seam approach for this ticket, which
a policy audit rejected under the no-fake-data rule; that code is not
reintroduced. Evidence README rewritten to describe the actual files.
11 passed in tests/test_shard_runtime_harness.py.
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