fix(proxy): reconcile durable full resend owner

[Soju06]

The fresh durable full-resend path can still fail before upstream dispatch when a broad legacy session-header sticky row points at a different account than the durable bridge owner: codex-lb returns continuity_owner_conflict as a retryable 503, the client repeats the identical request, and neither persisted owner ever changes — a deterministic retry loop even though the request already carries fingerprint-verified complete context.

This change represents complete durable full-resend eligibility as a sealed, immutable, request-local proof (_VerifiedDurableFullResend) created only by the count/fingerprint/retained-output or response-bound pending-tool-call verifier. For that proved fresh reattach only, the broad session-header alias is dropped from account selection and the upstream handshake, while the durable canonical key and durable owner account remain required. The broad sticky row is left intact for sibling traffic; specific turn-state/previous-response/file owner conflicts remain fail-closed. See openspec/changes/reconcile-durable-full-resend-owner/ for the full design.

Supersedes #1484 — original implementation by @mhooooo, taken over per the 07-30 maintainer notice. The author's commit is preserved with original authorship; takeover fixes sit on top.

What survived vs. what changed in the takeover

Survived unchanged (author's commit, rebased onto current main):

• The sealed _VerifiedDurableFullResend proof, including the round-4 restack requirement (responses_input_suffix_matches_pending_tool_calls settlement path) — the author had already addressed it before going silent.
• The owner-bound broad-session reconciliation (fresh_reattach_broad_session_owner_ignored), affinity-header stripping on session creation, the anchor-injection fallback for proof-failed full resends, the openspec change, and the author's unit + integration tests (including the real, unstubbed test_backend_responses_verified_full_resend_ignores_stale_broad_owner_on_durable_account).

Dropped:

• The three fix(proxy): avoid stale anchor on durable full resend #1481-shape dependency commits are no longer in the series — they were superseded by what already landed on main (fix(proxy): release idle bridge sessions' account stream leases #1476/fix(proxy): spill unanchored forks on account caps #1499 et al.), so the branch carries only the single implementation commit.

Takeover fixes on top (reconciliation with #1486/#1394/#1558/#1527/#1531/#1532 and the codex P1):

• Restacked onto main's classify_durable_full_resend structure and the post-fix(http-bridge): stabilize silent and clean-close recovery #1394 recovery-attempt journal flow; the unit test's account-neutral classifier assertion is now conditional because main computes neutrality eagerly for the recovery journal on the preserved path. None of the reconciliation was subsumed by fix(http-bridge): stabilize silent and clean-close recovery #1394 — main still leaves the broad session alias in selection for a proved full resend, so the conflict loop remained live.
• Updated the stale anchor-injection comment (injection can now also run for a full resend that failed proof verification).
• Addressed the open codex P1 on fix(proxy): reconcile durable full resend owner #1484 (the unit regression test pre-binds the session and stubs _get_or_create_http_bridge_session/_submit_http_bridge_request): added bridge-level coverage test_backend_responses_verified_full_resend_fails_over_to_new_account_after_owner_loss — a real turn completes on account A through the actual create/response-create gate, A is paused, and the verified session-header full resend fails over to account B with the complete history and no previous_response_id.
• Reworded the modified openspec requirement so the MUST clause sits on the first body line, per the current validator.

Gates

• architecture-check: pass (no ratchet changes)
• test-unit: 5283 passed, 3 skipped
• test-integration-bridge (test_http_responses_bridge.py + test_proxy_websocket_responses.py): 220 passed
• ruff check / ruff format --check: clean
• openspec validate reconcile-durable-full-resend-owner: valid

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