feat: add isCancelled support to download_to_file

Check cancellation callback after each 8KB block during body read.
Allows callers to abort long downloads promptly (e.g. ESC in TUI).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

feat: initial release of mcpplibs-tinyhttps

Minimal C++23 HTTP/HTTPS client library extracted from llmapi.
Features: HTTP client, SSE streaming, proxy support, mbedTLS TLS.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>