feat(ffi): let a host ask which moss it loaded

[ForeverInLaw]

A host resolves the shared library by path at runtime — mosh via moss-runtime/, mosh-org via --moss-lib/MOSH_ORG_MOSS_LIB, the probe from the directory beside the binary. So an old library sitting next to a new host is an ordinary mistake, and its symptoms are transport bugs the host has no way to attribute to the library.

Nothing exposed the version: not the FFI surface, not GetMeshInfo.

Moss_Version returns the build stamp as a C string (free with Moss_Free). The release workflow now passes -ldflags "-X main.buildVersion=<tag>"; anything built another way reports "dev" rather than claiming a version it cannot know. Callers should treat a missing symbol as "older than v0.8.17".

New symbol only — no existing signature changes, so every current host keeps working untouched.

Tests

• the default stays "dev", so a hand-built library never claims a release;
• the linker stamp survives into a real c-shared build — checked against the built artifact, not a test binary, because that is the only place the flag actually applies.

The cgo-in-_test.go route is not available (Go rejects it outright), so the symbol's body is a one-line wrapper over the variable both tests pin.