statsig-go: Fix C — JNI-style callback FFI for raw_feature_gate

[nsaini-figma]

Summary

Prototype of the structural fix for the labmate SIGSEGV in the
Rust→Go FFI string return path. Companion minimal-fix prototype
(Fix A+B — NUL-scan + ptr guard) lives on branch
nsaini/fix-a-nul-scan. Both prototypes are draft for side-by-side
review.

What changed

• New #[no_mangle] pub extern "C" fn statsig_get_raw_feature_gate_cb
  in statsig-ffi/src/statsig_c.rs. Builds the JSON string inside
  the closure body of use_raw_feature_gate_with_options and passes
  it to the Go callback by borrowed slice. The existing
  statsig_get_raw_feature_gate extern is left untouched.
• statsig-go/statsig_ffi.go: registers the new callback symbol via
  purego.
• statsig-go/statsig.go GetFeatureGateWithOptions: rewritten
  to use the callback path. Other gate/experiment/config functions
  are unchanged — they still use the heap-transfer model. Follow-up
  work would port them once Fix C is validated.
• statsig-cpp/include/libstatsig_ffi.h, statsig-ffi/include/statsig_ffi.h,
  statsig-dotnet/src/Statsig/StatsigFFI.g.cs: auto-regenerated by
  cargo build to reflect the new extern. No hand edits — included
  to keep the checked-in bindings in sync with the Rust ABI.

Why this shape

Mirrors the JNI binding (statsig-ffi/src/jni/statsig_jni.rs:838-861)
and pyo3/napi: the Rust side builds the host-language object inside
the closure while the borrow is alive. No *mut c_char ever crosses
the FFI boundary, no *mut u64 inout writeback, no free_string
round-trip — i.e. every failure surface identified for the original
crash is eliminated for this code path.

Failure modes covered

• bad len from purego trampoline → not used
• bad ptr from purego trampoline → buffer is Rust-owned, only crosses
  the boundary for the callback's lifetime
• premature free → no heap transfer to free
• internal race → same Rust RwLock as before

Test plan

• cargo check -p statsig_ffi clean
• cargo build --release -p statsig_ffi --target x86_64-unknown-linux-gnu clean
• nm -D libstatsig_ffi.so | grep statsig_get_raw_feature_gate_cb confirms symbol exported
• go vet ./statsig-go/... clean
• go build ./statsig-go/... clean
• Validation in labmate staging once tagged as v0.19.4-figma2

Refs

• Investigation: ~/nsaini/state/explorations/labmate-statsig-ffi-crash.md
• Fix design: ~/nsaini/state/explorations/labmate-statsig-ffi-fix.md (Fix C section)
• Companion PR (minimal): branch nsaini/fix-a-nul-scan