Harden capture, transactional history and release workflows for 2.1.0

[jakezwang]

This prepares Argon 2.1.0 by fixing data identity, capture and mutation boundaries before expanding real-data use. BSON IDs retain their types; captured updates require exact images; native capture waits for a durable readiness barrier and survives restart; merge, undo and head changes commit atomically. Snapshot publication and GC share a persistent lock so a referenced object cannot be deleted concurrently.

Anonymous demos no longer expose native MongoDB connection strings. The API binds to loopback by default, requires explicit authentication for public control planes and checks browser origins. The embedded console now walks through two proposals from one pin, a real conflict, explicit review and undo, with local handoff, capture health and opt-in fixed-vocabulary completion events.

The CLI adds doctor, collection preparation and usable JSON results. CLI/API/package/MCP versions share 2.1.0 metadata, and release jobs explicitly order binary publication, npm publication and MCP registration. Driver compatibility checks, executable examples and the companion SDK/browser/benchmark suites cover the intended workflow. Reachable dependency advisories are addressed with compatible driver/network patches and Go 1.26.6.

Validation on commit ba6e06a9a3b31124d6c37475b5667dd70ab42379: the complete core suite passed (219 tests/subtests, MongoDB/filesystem/S3 included), focused correctness and concurrency regressions passed with the race detector, and API/CLI suites passed. All three modules passed vet and reported zero reachable vulnerabilities in the current official govulncheck database. The final embedded console passed real browser review/merge/undo checks. Five release targets cross-compiled cleanly; native macOS and the actual packed npm installer/wrapper reported 2.1.0, with verified SHA256 sums. Python 3.10/3.12/3.14 each passed eight live-stack SDK tests. Remote CI run 34148948970 passed on the same commit: core/CLI/API tests, driver compatibility, all three vet jobs and all three reachable-vulnerability scans (8/8 checks). Publication remains a separate operation.

Operational limits remain explicit: stop native writers before destructive lifecycle operations; coordinate those operations with one control-plane owner; unsupported DDL or missing images degrades capture; an abandoned snapshot lock requires stopped-worker recovery. Serial transactional publication reduces concurrent throughput on a single branch; timing remains reported, correctness assertions remain mandatory, and absolute speed floors are opt-in. See docs/RELEASE_2_1.md.