fix: retarget updater to @mosaic/mosaic (#384)

docs/scratchpads/update-checker-package-20260404.md

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+# 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.
+
+## Validation
+
+- `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`
+- pre-push hooks: `typecheck`, `lint`, `format:check`
+
+## Review
+
+- 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.
+
+## Risks / Notes
+
+- Direct `pnpm --filter @mosaic/mosaic typecheck` and `pnpm --filter @mosaic/cli ...` checks were not representative in this worktree because `packages/cli` is excluded from `pnpm-workspace.yaml` and the standalone package check lacked the built workspace dependency graph.
+- The repo's pre-push hooks provided the authoritative validation path here and passed: root `typecheck`, `lint`, and `format:check`.