feat(request): refactor to thenable handle with lifecycle callbacks

[cc-hearts]

Summary

Refactor `Request` return type from state object to `RequestHandle` — a thenable that can be directly awaited, with lifecycle callbacks as an alternative API surface.

Breaking changes

Before	After
`await api.get('/u').promise`	`await api.get('/u')`
`result.data`, `result.error`, `result.loading`	removed — use await value / try-catch / callbacks
`result.aborted`	removed — check `AbortError` name or use `onAbort` callback
Promise always resolves with `T	null`

New features

• Lifecycle callbacks: `onSuccess`, `onError`, `onAbort`, `onFinally` on `RequestConfig`
• Timeout: per-attempt timeout via `RequestConfig.timeout`
• Retry: automatic retry with `retry` / `retryDelay`
• Cache: in-memory GET response cache with TTL
• Deduplication: concurrent GET requests share the same inflight promise
• Download progress: `onDownloadProgress(loaded, total)` callback
• Richer body support: `Blob`, `URLSearchParams` pass through without JSON.stringify
• Full response type parsing: `204/205 → null`, binary → `Blob`, uses `MIME_TYPES` constants

Docs

• JSDoc with composable best practices on `Request` class (English)
• `README.md`: "Request — composable best practices" section (English)
• `README_ZH.md`: Chinese version with same content + config reference

Tests

36 tests total (19 new), all passing.