CallStackCapture now delegates capture and symbolization to StackTrace instead
of execinfo plus an addr2line subprocess. The same implementation is enabled on
Linux and Android. Reports contain the module, module-relative offset and
demangled function; dladdr supplies no source line.
StackTrace captures the calling thread with _Unwind_Backtrace, symbolizes
addresses with dladdr and __cxa_demangle, and samples other threads through a
real-time signal. Each target writes into its own slot; the sampler also reads
/proc task name, scheduler state and wait channel, and keeps unanswered threads
in the result. Windows captures stacks and module offsets but reports that
remote sampling is unsupported. Debugger detection is exposed separately.
