feat(typescript): add TypeScript namespace client and align architecture with Python

[Xuanwo]

This PR follows the python's pattern and added a ts namespace client.

Key design points

• core package only provides abstractions and class-path based implementation loading
• NATIVE_IMPLS follows Python-style aliases (rest, dir) and dynamic loading
• REST implementation lives in a separate package and is wired through class paths
• OpenAPI generation is post-processed by typescript/scripts/patch-fetch-client.mjs to keep build/lint/test and exports stable after regen

Validation

• make -C typescript build
• make -C typescript lint

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Parts of this PR were drafted with assistance from Codex (with gpt-5.3-codex) and fully reviewed and edited by me. I take full responsibility for all changes.