docs: explain in-memory programmatic builds

[ColumbusLabs]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.

Description

Documents the Programmatic Usage write:false item from #247 across the English guide, Chinese guide, and synchronized tsdown skill reference.

The new section shows an in-memory build, explains chunk and asset content access, documents the current TsdownBundle[] return shape, notes remaining filesystem side effects, and calls out the watch-mode incompatibility. It also corrects the skill reference’s stale Promise description.

This is AI-assisted documentation, not core code. The final diff was validated against the implementation with a live runtime probe and independently reviewed.

Validation:

• live build({ write:false, clean:false }) probe returned an in-memory bundle
• pnpm lint
• pnpm typecheck
• pnpm docs:generate
• pnpm docs:build
• targeted Prettier and git diff --check

Linked Issues

Addresses the unchecked Programmatic Usage write:false documentation item in #247.

Additional context

The docs explicitly avoid combining write:false with watch mode because current watch rebuilds do not expose returned bundles.

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commit: 1ad9f93

[sxzz]

Thanks!