[choiyounggi]

Refs #228, #229. Tracking: #255 (P0 group).

What this does

The only production DB driver (lnpl-postgres) implements none of the optional RepositoryDriver methods for idempotency claims (idempotency_begin/finish/release) or outbox drain (drain_outbox/ack_outbox). Before this PR the core selected those features by hasattr, so on Postgres an Idempotency-Key was silently ignored (#228) and lnpl outbox drain / lnpl relay died with an AttributeError (#229). This PR makes the capability explicit and makes both consumers fail loudly.

commit	change
fc74d99	RepositoryDriver.supports_idempotency() / supports_outbox_drain() — introspection default (True iff every method of the set is callable; Sqlite True, external drivers lacking the methods False, no base-class stubs). RepositoryDriverTCK gains 6 declared-unsupported cases (no skipTest; a driver that reports False must have the methods absent). docs/backends.md capability table.
c251bd3, 192c0fc	lnpl outbox drain / outbox ack / relay: check the predicate before any drain/ack/open_publisher call; False → one stderr line error: backend <scheme> does not implement the outbox drain contract (drain_outbox/ack_outbox) — see docs/backends.md §3, driver closed, rc 2, empty stdout. --backend fake keeps its existing rc 2 refusal. cli-surface.md updated.
6430ba3	lnpl serve / build_app(): a RepositoryDriver whose predicate is False → requests carrying Idempotency-Key (M22) and every POST /-/events/<slug> delivery (E8) are refused with 501 idempotency-unsupported (problem+json), the workflow does not run; requests without the header run as before. One boot line on stderr (lnpl serve: repository driver <module.qualname> -- idempotency: unsupported …), GET /-/readyz 200 body carries "capabilities": {"idempotency": <bool>} (never a 503). Non-RepositoryDriver objects (test doubles) take an isolated legacy branch that preserves the previous behaviour. lexer.RESERVED_PROBLEM_CODES entry; docs/serving.md rows.
08196ac	CHANGELOG [Unreleased] entries; READMEs re-pinned to the measured suite size.

Decisions recorded during the run

• User decision (Postgres 백엔드에서 Idempotency-Key가 경고 없이 무시된다 — 드라이버 opt-in 미지원을 기동 시 감지하고 TCK에 멱등성 계약을 넣는다 #228): refuse with 501 rather than warn-and-run or refuse at boot.
• Predicates instead of base-class stubs (lnpl outbox drain/lnpl relay가 drain_outbox/ack_outbox 없는 드라이버(postgres)에서 AttributeError로 죽는다 #229 scope item 2): a stub would have made hasattr true for every driver and turned silent skips into 500s before the consumers switched.
• The 501 detail / boot line name the driver as module.qualname — no backend scheme string reaches wsgi.py on the lnpl serve path.
• Driver-side implementation of the five methods is lnpl-postgres#8 (separate run, after this TCK lands). With this PR, lnpl-postgres's linkly-sync latest-main job runs the 6 new TCK cases through the declared-unsupported branch (verified against its sources).

Verification

• Isolated worktree at 08196ac, own venv: Ran 5394 tests in 174.778s / OK (skipped=1) (mode B included); rfc_lint, gen_plugin_references --check, check_doc_snippets, check_version_sync, ruff check impl/lnpl scripts all rc 0. Doc-cell fix 192c0fc re-checked: gen_plugin_references --check rc 0, test_plugin_references/test_readme_currency/test_cli_relay Ran 106 tests … OK.
• Per task: independent task-reviewer approve ×4 (no blocking findings), test-quality-auditor cross-call PASS ×3 (mutation probes: predicate return True/any, old hasattr selection, guard removal — all caught), integration-reviewer approve.

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https://claude.ai/code/session_015TZZN39cJVjPsK2Ha3C5LD