fix: replace deprecated <strstream> with <sstream> in porthopper example

[mvanhorn]

Summary

In protocol.hpp, replace #include <strstream> with #include <sstream>. Replace each std::istrstream is(ptr, len) with std::istringstream is(std::string(ptr, len)), and each std::ostrstream os(data_, N) with a local std::ostringstream os, followed by os.str().copy(data_, N) (after checking os state) so the fixed data_ buffer receives exactly the same setw-padded bytes as before; no interfaces of control_request or frame change, so client.cpp and server.cpp need no edits and serve as the compile/behavior surface. Graveyard delta vs closed PR #1729: that PR was auto-generated by a bot app and was closed unreviewed with zero comments (the issue author himself asked what happened to it); this plan is a human-authored minimal diff that avoids #1729's raw std::memcpy (which requires a <cstring> include and risks copying fewer bytes than the buffer on a failed stream) by using std::string::copy with explicit stream-state and length checks, and it documents that the on-wire format is unchanged.

Why this matters

The C++11 porthopper example includes <strstream> and uses std::istrstream / std::ostrstream in src/examples/cpp11/porthopper/protocol.hpp. <strstream> has been deprecated since C++98 and is removed in C++26 per P2867R1, so GCC 16 emits -Wdeprecated-declarations and #warning noise at lines 56, 65, 84, 115, and 124, and the example will stop compiling under C++26. The reporter (heitbaum, LibreELEC) hit this building the examples with a GCC 16 toolchain. The fix is to switch the header to <sstream> with std::istringstream / std::ostringstream while keeping the fixed-width, space-padded wire encoding byte-identical.

See #1725.

Testing

• Happy path: build the cpp11 porthopper example (client.cpp, server.cpp) with a current GCC/Clang at -Wall; compiles with no deprecated-header or deprecated-declaration warnings. - Round-trip: run server and client locally; frames and control requests decode the same port/frame numbers that were encoded, confirming the space-padded fixed-width encoding is byte-identical to the strstream version. - Edge cases: encode port 0 and 65535, frame number 0 and a large value; decoded values match and encoded fields remain exactly encoded_port_size / encoded_number_size bytes. - Error path: stream failure during encode (value too wide for the field) leaves the object in the same defined state as before; decode of short/garbage input fails gracefully as it did with istrstream.

Fixes #1725