fix(0088): reject dataflow cycles in construction build (closes #161)

The construction op-script service accepted a dataflow cycle and exited 0,
emitting a non-DAG blueprint — violating domain invariant 5 / C9 (the graph is a
DAG; the only feedback path is an explicit delay/state node). The cycle-0088
fieldtest surfaced it (fieldtests/cycle-0088-construction-op-script/FINDINGS.md).

The construction gate set had no acyclicity check: a cyclic interior-edge set
passed both the eager per-op gates and the holistic finish, so replay() returned
Ok. Fix: an eager reachability check at the connect op that closes the cycle —
GraphSession::closes_cycle(from, to) asks "does the `to` node already reach the
`from` node through the edges added so far?" and rejects with a new by-identifier
OpError::WouldCycle { from, to } ("connecting from -> to would close a cycle"),
threaded through the CLI's exhaustive format_op_error. The fault is attributed to
the offending connect op, consistent with the surface's eager per-op diagnostics.

Eager rather than a holistic topological check at finish: a cycle is closed by
one specific connect, and naming that connect is the sharpest by-identifier error.

RED-first: two tests pin the symptom (a 2-cycle a<->b and a self-loop), both red
before the fix and green after; the 19 existing construction tests, the full
workspace suite, and clippy stay green.

Scope is the cycle only. The validity-floor cases the fieldtest also noted (an
unfed declared source role, an output-less graph) are degenerate-but-well-formed,
not invariant violations, and are deferred (refs #161) pending the build->consume
edge (#28) that defines what a valid emitted blueprint must guarantee.

crates/aura-cli/src/graph_construct.rs

@@ -85,6 +85,7 @@ fn format_op_error(e: &OpError) -> String {
             format!("kind mismatch {from} -> {to} (producer {producer:?}, consumer {consumer:?})")
         }
         OpError::SlotAlreadyWired { node, slot } => format!("slot {node}.{slot} already wired"),
+        OpError::WouldCycle { from, to } => format!("connecting {from} -> {to} would close a cycle"),
         OpError::BadParam { node, err } => format!("bad param on {node}: {err:?}"),
         OpError::Incomplete(ce) => format!("{ce:?}"),
     }