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.