fix(lastcode): keep checkpoint migrations replayable

[lastobelus]

A nightly replay gave upstream and LastCode the same migration number, leaving checkpoint 1185 with a manifest that referenced missing renumbered files. The checkpoint smoke gate did not typecheck the server, so that invalid tree was published.

This reserves upstream migration 42 in LastCode source, shifts the LastCode migrations to 43-47, and adds an idempotent migration 48 bridge for databases that already recorded the old LastCode IDs. Projection upserts accept the two fields absent from pre-annotation events, while reads remain strict. The checkpoint smoke gate now runs the migration compatibility regression and typechecks the server before publication.

Validation:

• pinned Node 24 quick local CI
• 41 focused tests on the generated checkpoint-1185 replay tree
• server typecheck on the generated replay tree
• exact same-nightly replay with rerere auto-staging and daemon-style continuation

Implemented with GPT-5.6 Codex in T3 Code.