fix(planning): refuse answers on superseded and archived interview questions and link the live question #9715

Parent

Refs #5796

Depends on

• none: on the frontier now

Outcome

An archived or superseded question takes no decision and points at the live end of its chain (gap G4; also
#5795 "Archived and superseded questions keep live controls", which its triage assigned here):

• The page disables the decision choices on archived and superseded questions, keeps Ask, and adds a primary
  "Go to Q61" button naming the last live question of the supersededBy chain.
• The server answers 409 {"error": "superseded", "by": "Q61"} to a decision on one.
• The page's state check reads supersededBy before the decision, so an answered superseded question no longer
  counts as answered and live in the meter, and export-ledger never drops an answer the page counted live.

Key interfaces

• Interview page: plugins/planning/surface/index.html; page server: plugins/planning/surface/server.py
  (record(), derive_states); round CLI: plugins/planning/surface/round.py behind round.sh;
  exporters: plugins/planning/surface/exporters.py.
• The interview skill's surface contract, plugins/planning/skills/interview/context/surface.md, documents
  every new op, field, setting and refusal.

Done when

• A server test posts a decision on a superseded question and gets 409 with "by" naming the chain's live end (two hops away in the fixture).
• A page test shows disabled decision choices, an enabled Ask and the "Go to" button on an archived question.
• A test answers a question, supersedes it, and checks the meter count and the export-ledger output agree.
• scripts/run-test-file.sh plugins/planning/surface/test_server.py exits 0.
• scripts/run-test-file.sh plugins/planning/surface/test_round.py exits 0.
• bash plugins/planning/surface/surface.test.sh exits 0, so a data dir without the new fields still loads, renders and exports.
• scripts/check-changelog-fragments.sh --check-required origin/main exits 0.

Out of scope

• The rest of fix(planning): interview page visual audit follow-ups to #5569 #5795's visual findings.