chore!: migrate build pipeline from tsc to tsdown

[chybisov]

Which Linear task is linked to this PR?

N/A — Build tooling improvement.

Why was it implemented this way?

Migrates the build pipeline from tsc to tsdown (powered by Rolldown/OXC in Rust) for significantly faster builds. This approach was proven in the SDK repo where build times went from ~13s to ~650ms.

What changed:

1. tsdown added as root devDependency — single build tool replacing multiple tsc invocations per package.

2. tsdown.config.ts created for each package — dual ESM/CJS output with source maps and declaration files (.d.ts with source maps via OXC's isolated declarations).

3. isolatedDeclarations: true added to root tsconfig.json — required by OXC's declaration emitter. All exported functions, variables, and class members now have explicit type annotations (~30 files updated).

4. Package scripts simplified:

   • build inlined to single command (rm -rf dist && version.js && build.js && printf package.json markers)
   • Removed build:cjs, build:esm, build:types, build:clean scripts
   • clean simplified to rm -rf dist
   • watch replaced with tsdown --watch
   • check:types (tsc --noEmit) kept as-is

5. scripts/build.js created — per-package build timing wrapper around tsdown.

6. All packages now produce both ESM and CJS — previously only @bigmi/core had CJS output.

Why tsdown over alternatives:

• Built on Rolldown (Rust) — dramatically faster than tsc
• Native isolatedDeclarations support via OXC — generates .d.ts without a full TypeScript program
• Drop-in replacement that preserves the same output structure (dist/esm/, dist/cjs/)

Visual showcase (Screenshots or Videos)

Build output after migration:

✔ @bigmi/react built in 531ms
✔ @bigmi/client built in 739ms
✔ @bigmi/core built in 871ms

Checklist before requesting a review

• I have performed a self-review and testing of my code.
• This pull request is focused and addresses a single problem.
• If this PR modifies the Bigmi API or adds new features that require documentation, I have updated the documentation in the public-docs repository.