Mirrors the 2-node TestCrossLeaf_SharedProjectCode_PropagatesCommit shape but with a third leaf attached to the same topology root. Asserts that a commit on any peer propagates to BOTH of the other two peers, both directions.

Reproduces the cross-leaf propagation gap reported in #179 and confirms its underlying cause is the multi-responder .sync race filed as #162: with 3+ hubs subscribing to the same .sync subject every peer answers nats.Request, only the first reply wins, and a stale responder silently starves the pull. The first hop (B -> A) is already enough to surface this — A's .sync request can race against C's empty responder even though A and B are the only nodes that need to talk for that direction.

Gated under -short with a skip pointing to #162 so pre-commit and CI stay green. Without -short the test fails at the 10s wait in ~10s, demonstrating the bug. Remove the skip once #162 is fixed; the test then doubles as the regression sentinel for the 3+ hub topology that production sesh relies on.

No production code changes.

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