feat(modelchecker): expose action returns on Link + add no-state-retu…

[jp-fizzbee]

…rns flag

Builds on the prior CloneWithRefs fix by giving downstream consumers (graph, MBT, explorer) a forward-compatible way to read action return values from the per-transition Link rather than per-state Process.Returns.

Producer-side additions:

• New Link.Returns starlark.StringDict field, populated in processNode alongside Labels/Messages and propagated by Attach/Duplicate to outbound links.
• proto/graph.proto Link gets repeated NameValue returns = 10 so the per-transition returns are serialized into adjacency-list .pb files. Sorted by name in returnsToNameValues for deterministic output.
• GenerateDotFile renders the return value on the edge label as \n returns: {...}.

Migration flag:

• New --experimental_no_state_returns (off by default). When on, suppresses Process.Returns from HashCode, MarshalJSON, and the dot-file state label. Returns remain populated during execution (functions/invariants still read them) and remain visible on Link.Returns. Use this to verify downstream consumers have migrated; planned to become the default once they have.

Drive-by reverts unrelated to Returns work:

• main.go: dotfile threshold local change 500 -> 250 (local-only bump that caused 11-07 and 13-02-01 reference baselines to drift).
• processor.go: printPeakSummary now suppressed under the default queue mode and only prints under --experimental_processed_queue. The metric is only meaningful for old-vs-new dedup comparison; printing it in the default path drifted every reference baseline by one line.

Verified: all 94 reference specs + 10 lovable specs pass.