Investigate migrating from unmaintained tsup to tsdown

[cliffhall]

Background

The CLI and TUI clients (and test-servers) currently bundle with tsup, which is effectively unmaintained. tsdown is the actively-maintained, Rolldown-powered successor positioned as a drop-in replacement.

Goal

Investigate why we're on tsup and evaluate migrating to tsdown.

Tasks

• Inventory every place tsup is used (clients/cli, clients/tui, test-servers, any others) and the config/options each relies on.
• Assess tsdown compatibility with those configs (entry points, formats, dts, externals, the @inspector/core alias, watch mode).
• Prototype the migration for one client and confirm builds + tests pass.
• Document findings and recommend whether/how to migrate.

Acceptance criteria

• A clear recommendation on migrating to tsdown, with the migration path and any blockers identified.