Native C/C++ (NDK) crash capture, manual and automatic

[ioannisj]

Extracted from PostHog/posthog#43012.

Problem

There's no native code in the SDK: no signal handlers, JNI, or .so symbol handling. Capture is limited to JVM Throwables (manual plus the uncaught handler).

Scope

• Native signal-handler capture (SIGSEGV, SIGABRT, and friends), a manual native capture API, symbol upload, and the server-side symbolication path.

Large, probably an epic rather than a single PR.