Scratchpad — updater package target fix (#382)

Objective: Fix mosaic update so modern installs query @mosaic/mosaic instead of stale @mosaic/cli.
Scope: updater logic, user-facing update/install hints, tests, package version bump(s).
Constraints: preserve backward compatibility for older @mosaic/cli installs if practical.
Acceptance:
- fresh installs using @mosaic/mosaic report latest correctly
- older installs do not regress unnecessarily
- tests cover package lookup behavior
- release version bumped for changed package(s)

Decisions

Prefer @mosaic/mosaic when both modern and legacy packages are installed globally.
For legacy @mosaic/cli installs, query @mosaic/cli first, then fall back to @mosaic/mosaic if the legacy package is not published.
Share install-target selection from packages/mosaic so both the consolidated CLI and the legacy packages/cli entrypoint print/install the same package target.
Extend the update cache to persist the resolved target package as well as the version so cached checks preserve the migration target.

pnpm install
pnpm --filter @mosaic/mosaic test -- __tests__/update-checker.test.ts
pnpm exec eslint --no-warn-ignored packages/mosaic/src/runtime/update-checker.ts packages/mosaic/src/cli.ts packages/mosaic/src/index.ts packages/mosaic/__tests__/update-checker.test.ts packages/cli/src/cli.ts
pnpm --filter @mosaic/mosaic lint

Manual review of the updater diff caught and fixed a cache regression where fallback results would lose the resolved package target on subsequent cached checks.

pnpm --filter @mosaic/mosaic typecheck fails in this worktree on pre-existing unresolved workspace imports such as @mosaic/quality-rails, @mosaic/prdy, and @mosaic/types; this is broader than the updater change.
packages/cli is excluded from pnpm-workspace.yaml, so package-script lint/typecheck are not runnable via normal workspace filters in this worktree.