binaries-linux-gnu: add Go-module skeleton

[nsaini-figma]

Summary

Adds the per-platform binaries Go-module skeleton (binaries-linux-gnu/{go.mod, linux_gnu_x86_64.go}) that future release tags will drop a rebuilt libstatsig_ffi.so into. Mirrors the upstream statsig-io/go-server-core-binaries-linux-gnu shape so the wrapper's import resolves under a consumer-side replace directive.

Why

This fork needs to ship a rebuilt .so carrying fixes that haven't landed upstream yet (including the exposure-dedupe memory leak tracked in upstream PR statsig-io#47). To distribute it via Go modules (matching upstream's existing distribution mechanism), we need a fork-owned Go module that embeds the rebuilt binary. This PR is the bare module shape; the .so itself is dropped on tag-reachable commits by the release workflow (separate PR).

What changed

• binaries-linux-gnu/go.mod: declares module github.com/figma/statsig-server-core/binaries-linux-gnu.
• binaries-linux-gnu/linux_gnu_x86_64.go: //go:embed linux_gnu_x86_64.so plus GetBinaryData() and GetSignatureData() shims.

Invariants worth calling out

• package go_server_core_binaries_linux_gnu (with underscores): MUST match upstream's package name; the wrapper resolves the package by symbol, not URL.
• .so is intentionally NOT in this PR. At HEAD on main, this module won't compile, which is expected. It only becomes a valid Go module at tagged release commits (created by the release workflow in the follow-up PR).

Action required from a repo admin before merge

These are admin-only and cannot be done in this PR:

• Workflow permissions (Settings → Actions → General → Workflow permissions): set "Read and write permissions" so the upcoming release workflow's `permissions: contents: write` block is honored.
• Tag protection (Settings → Tags → Protection rules): ensure no rule blocks `statsig-go/v*-figma*` or `binaries-linux-gnu/v*-figma*` from being pushed by `GITHUB_TOKEN`. If protection is required, authorize the workflow's bot identity.

Test plan

• Confirm files exist on the branch (`git ls-tree HEAD -- binaries-linux-gnu/` shows go.mod plus linux_gnu_x86_64.go, no `.so`)
• Confirm package declaration is `go_server_core_binaries_linux_gnu`
• No CI test runs against this PR (no build expected to succeed at HEAD)

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com