The reconstruction runs [`forward_chain`] over a **placeholder source buffer** of zero bytes (length matching the `Source` op's declared shape × dtype). Forward execution honours fusion, role overrides, and every op's actual descriptor flow, so the descriptors returned here match exactly what an end-to-end forward pass on the real tensor would produce. The decoder calls this to feed faithful `Plane` values into [`inverse_chain`]. Cross-tensor delta ops are out of scope here: the placeholder forward would need real dep planes. Chains containing `XorDelta` / `FloatDelta` cannot reconstruct terminal descriptors via this helper alone — the compressor driver owns that orchestration.

Node 0 must be `Source`; its output wraps `ctx.source_bytes` in a `Plane`. Each subsequent node comes from `op_from_id`, and the runtime calls `forward` with the gathered input planes. Terminal refs collect at the end in `chain.terminals` order.

`source_bytes` is borrowed: the runtime owns the single allocation that seeds node 0's `Plane`, sparing callers (e.g. multi-chain trial-encode) from cloning the raw tensor for every candidate.

The traversal walks nodes in reverse topological order. For each node, `op.inverse` runs on the node's output planes (from the working state) to reconstruct the input planes, then stores them under the corresponding source edges. After the traversal, the plane at `(0, 0)` is the reconstructed source.

Conditions for fusion: 1. The connecting edge has no `role_override` and empty `vendor_bytes`. 2. No other edge or terminal consumes the producer's output. 3. The (producer.op, consumer.op) pair is in the fusion table.