fix: settle native Cursor setup failures

[lidge-jun]

A native Cursor setup failure could emit compatibility completion before the run was admitted to Activity and leave its durable trace header running. Acquisition errors bypass normal lifecycle settlement, so canonical termination alone did not close the header.

Reuse one fallback projection to emit start before compatibility completion, then attempt canonical termination and independently close only the captured still-running trace. The conditional update preserves any result already settled before a later cleanup failure. Diagnostics remain separate from an absent model final; no assistant MESSAGE is fabricated.

Validation: an owned Node ACP peer completes initialize/authenticate then returns exact session/new error -32603 through the real main runtime, SQLite and scoped HTTP SSE. Regression cases cover duplicate replies, ready/settle exceptions, journal/listener faults, stale owners, cancellation, finalizer exceptions and late release. Header and event-order REDs became GREEN. Independent Astra A/B/C reviews passed after strengthening the exact-error and complete-SSE assertions. Final head 40d235d6be4e6de2f058a11d5c8b84e374d311cb passed 327 focused tests across 28 explicit files, types and 487 documentation counts. Builds, frontend types, assets and strict gates also passed on the identical production tree. Exact-head Tests and Postinstall Platform Checks both succeeded with all jobs green.

Stack: based on #546 (codex/native-activity-journal). Source-only commit: 4c5c061f4a3767f978e5d16a6daf16c27aa85b69. No UI or provider SDK changes, release or deployment.

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[lidge-jun]

Closing: this PR's content is absorbed by #551 (codex/native-activity-dev-reconcile), the single cumulative candidate for dev.

The same Cursor setup-failure settlement already landed in dev via #542 and is reconciled in #551.

Verification (fetched 2026-09-07): every non-devlog file this PR changes versus its merge-base with dev has byte-identical content in the #551 tree, except files renamed or superseded by a later layer (#552–#559). No unique change is lost. The branch is kept.

[lidge-jun]

Correction (2026-09-07, post-merge audit): the earlier close comment overstated equivalence. After #559 landed, a fresh check against origin/dev shows this branch's commits are not ancestors of dev — #551 re-authored the donor content as integration commits — and many files diverged through later layers. The functionality is in dev; the original authoring history is only here. This branch is intentionally kept as history, and is excluded from the post-merge branch prune. Do not delete without a separate decision.