design(arc): resolve Step-2 reference arc semantics

[pentaxis93]

Summary

• Adds the Step-2 reference arc design note for submit -> review -> land.
• Grounds review re-fire on runa on_change("change-proposal") semantics and makes review the authoritative producer of review-findings classification.
• Names the current substrate gaps for forge-operation resolution and approved-disposition land gating so feat(contracts): submit C-2 workflow contract (forge-invariant) #330-feat(sourcehut): SourceHut forge-tagged mechanics and submit->review->land dogfood #334 do not assume missing runtime/conformance behavior.

Changes

• Documents the review -> revise -> re-review cycle as immutable change-proposal.version rounds reviewed through review-findings.against_version.
• Records that classification lives in review-findings as produced by review, with no separate triage protocol in Step 2.
• Defines the operation resolution mechanism needed for forge-tagged mechanics and calls out that the Step-1 substrate does not yet implement active forge_tag selection or operation binding.
• Adds explicit source references and deliberate deferrals.

GitHub Issue(s)

Closes #329

Test plan

• python -m tooling.conformance
• python -m unittest discover -s tests
• rg -n "TBD|TODO|open question|assumption|left open" docs/architecture/step-2-reference-arc-design.md