docs(sdk): retry config, resumable downloads, UploadSessionExpiredError

[dobby-coder]

Closes #68.

Summary

• docs/sdk/js-encryption.md: new "Retry options" section documenting PostGuardConfig.retry (defaults: 5 attempts, 500 ms initial delay, 30 s cap, 2× multiplier, full jitter), per-phase timeouts (chunkTimeoutMs, finalizeTimeoutMs, downloadTimeoutMs), retry classification rules, the onRetry hook + RetryEvent shape, and the note that initUpload and finalizeUpload are deliberately not retried.
• docs/sdk/js-decryption.md: new "Retries and resumable downloads" section covering the Range-based resume, the 206 Partial Content requirement, the silent-rewind protection (200 OK on resume → fail-not-retry), the shared retry budget across resumes, and the v1.6 behaviour change where downloadFileWithRetry returns its ReadableStream synchronously.
• docs/sdk/js-errors.md: new UploadSessionExpiredError row in the hierarchy and a section with properties (uuid, reason, status 404, body) plus a usage example.

Source verification

• All symbols verified against encryption4all/postguard-js@a60716e (current main head).
• Source links pinned to that hash and curl -sI checked — all return HTTP 200, line ranges match the cited content (RetryOptions interface, downloadRange resume logic, UploadSessionExpiredError class).
• npm run docs:build succeeds; the new anchors render in dist/sdk/{js-encryption,js-decryption,js-errors}.html.

Reviewer quickstart

```
git fetch origin && git checkout docs/postguard-js-retry-and-resume && npm install && npm run docs:dev
```

Source PRs: encryption4all/postguard-js#47 (retry config + UploadSessionExpiredError) and encryption4all/postguard-js#52 (resumable downloads).