Add configurable per-node stall timeouts

[kurt-rhee]

What changed

Adds configurable stall timeouts for individual workflow nodes.

• Reads the optional stall_timeout node attribute.
• Uses a 3-hour default when the attribute is absent.
• Caps configured values at 72 hours.
• Updates the watchdog timeout whenever execution enters a new node.
• Keeps stall timeout reporting aligned with the node-specific value.

Why

Different workflow nodes can have very different expected runtimes. A single graph-wide timeout can terminate legitimately long-running nodes or leave short-running nodes waiting too long. Per-node configuration makes the watchdog behavior match each node's workload.

Validation

• Applied cleanly to Fabro revision 77e7704e815e9d5a46ebafad6d846085b1aeb148.
• git diff --check passed.
• Tests were not run in the source workspace per project instructions.