[M-Fatah]

Adds generic standard-stream byte I/O and byte-range string append overloads to Core.

• Add platform_stdin_read and platform_stdout_write across Windows, Linux, macOS, iOS, and Android.
• Return U64 byte counts, with U64_MAX for failures. Null buffers and zero-size requests are no-ops; reads return zero at EOF.
• Support partial transfers and cap requests to OS-supported sizes.
• Retry interrupted POSIX operations and handle closed output pipes without changing the process-wide SIGPIPE handler.
• Add string_append overloads for const char * with a byte count and Slice<const char>. Preserve embedded nulls and maintain the destination terminator.
• Update documentation and add byte-range append coverage.