feat(#1956): flag cross-artifact fact drift in the plan drift guard (#3259)

* test(#1956): failing-first contract for cross-artifact fact-drift pass

* feat(#1956): flag cross-artifact fact drift in the plan drift guard

* fix(#1956): correct config-key assertion and bidirectional lifecycle-lag exemption

* docs(#1956): document the cross-artifact axis in the architecture reference

* feat(#1956): decide the phase-status drift axis deterministically

* fix(#1956): scope the progress-table lookup, abstain without a position section, rank deferred

* docs(#1956): backfill changeset pr number

---------

Co-authored-by: sim <sim@local>

docs/ARCHITECTURE.md

@@ -792,6 +792,8 @@ remove or rewrite anything.
 
 The plan drift guard (`plan_review.source_grounding`) — which verifies symbol references in generated plans against live source before execution — is specified in [ADR 22](adr/22-plan-drift-guard.md).
 
+The same switch gates a second, cross-artifact axis: a fact-drift pass that compares the *same* fact as stated in `ROADMAP.md`, `PLAN.md`, `STATE.md` and `CONTEXT.md` and reports contradictions (a phase status, a success criterion, a requirement ID, a glossary term) with both locations and the authoritative side named. Where the source-grounding axis grounds a plan against code, this one grounds the planning artifacts against each other. It keys on contradicting knowledge rather than similar-looking text, and is advisory only — it never sets `hardBlock` and never contributes to the convergence counts.
+
 ### Platform Handling
 
 - **Windows:** `windowsHide` on child processes, EPERM/EACCES protection on protected directories, path separator normalization