DDIR: test shared-input joins through the server

[frankmcsherry]

DDIR: test shared-input joins through the server

Regression coverage only: no server code changes or new query capability.

Factor the existing integration test's process startup into a helper accepting
a worker count. The new scenario installs an arranged edge export and two
consumers that each join request rows (request_id, node) against it. Feed
requests and graph changes through TCP, then check peek after each tick.

This checks ordinary DD semantics through the server path: both consumers see
the shared input, retractions update the answers, equal bindings under distinct
IDs remain distinct, missing keys return no rows, and a reused ID gets the
answer for its new binding.

Four separately named tests run the same scenario on Vec and Corgi with one
and four workers, allowing parallel execution and identifying the failing
configuration directly. The README keeps only a short usage paragraph about
request rows and existing feed/tick/peek behavior.

Validation

From the worktree, using the existing pinned upstream Corgi:

CARGO_PROFILE_DEV_DEBUG=0 CARGO_PROFILE_TEST_DEBUG=0 CARGO_INCREMENTAL=0 cargo test -p ddir-server
CARGO_PROFILE_DEV_DEBUG=0 CARGO_PROFILE_TEST_DEBUG=0 CARGO_INCREMENTAL=0 cargo test -p ddir-server --test multiworker -- --test-threads=6
git diff --check c33b0fd0

The server suite passes, including the four backend/worker configurations and
the existing integration tests running concurrently. Existing workspace CI
discovers these tests. No new dependencies, data downloads, external database,
or performance thresholds.