ci: switch to StageX reproducible builds without QEMU

[ConYel]

Replaces the current golang:alpine Dockerfile with StageX — pinned
SHA256 base images, multi-stage quality gates (go vet, go test), hermetic
build with --network=none, deterministic timestamps, and native
cross-compilation via GOARCH build arg.

Key changes:

• .github/Dockerfile — StageX multi-stage, no QEMU, 13 architectures
• .github/workflows/ci.yml — per-arch shell loop, manifest annotation,
  no setup-qemu-action, no setup-buildx-action
• .github/verify-binary.sh — post-build ELF arch + version verification

Why:

• QEMU is ~20% slower and only builds 7 arches (StageX: 13)
• Current golang:alpine is a floating tag — not reproducible
• Current build installs git via apk, adds network, skips lint/test
• Go cross-compiles natively — no reason to use QEMU for building

Ref: #594