Migrate burl example to capy post-#262 buffer API

[mvandeberg]

cppalliance/capy#262 removes the slice_of<> class template and the slice CPOs (prefix, etc.). The burl example's any_stream type-erasure used both:

• The virtual async_write_some/async_read_some signatures took capy::slice_of<boost::span<capy::const_buffer const>>/mutable parameters.
• The outer rate-limited templates used capy::prefix(buffers, n) to limit each underlying call to the remaining byte budget.

Migration:

• Change the virtual signatures to take the underlying boost::span<X const> directly, plus a std::size_t max_bytes parameter carrying the byte budget. The implementation wraps the span in capy::buffer_slice(span, 0, max_bytes) and forwards s.data() to the inner stream.
• The outer templates pass buffers, wr_remain_ / buffers, rd_remain_ directly through the virtual; the slicing happens on the receiving side.
• Replace the <boost/capy/buffers/slice.hpp> include with <boost/capy/buffers/buffer_slice.hpp>.

The burl example remains disabled in CMake (pre-existing — its asio dependency requires a boost::asio::any_completion_handler.hpp that isn't in the pinned asio build). This migration keeps the source in sync with the new capy API so it can be re-enabled when the asio dependency is resolved.