Establish a single source of truth for the ONNX Runtime version

[Davidnet]

Context

In #406 we rolled the ONNX Runtime back to 1.24.2 and pinned github.com/yalue/onnxruntime_go to v1.27.0 because the C API headers in onnxruntime_go require a matching runtime version. The version is currently repeated in several places that can drift independently:

• go.mod (onnxruntime_go version, which implicitly dictates the runtime version)
• docs/02-development-guide.md (download URLs, cp targets, verify step, env var)
• The downloaded/copied shared library filename (libonnxruntime.1.24.2.dylib, libonnxruntime.so.1.24.2)
• Any CI / build scripts that fetch the runtime

Goal

Define a single source of truth for the ONNX Runtime version so the Go module, the shared library on disk, the docs, and CI cannot drift apart.

Possible approaches

• A Makefile / build-script variable (e.g. ONNXRUNTIME_VERSION) consumed by install scripts, CI, and a docs-generation step.
• A .tool-versions (or similar) entry, with docs referencing the variable rather than a literal version.
• A small helper script that downloads the correct runtime based on the pinned onnxruntime_go version.

Acceptance criteria

• Bumping the ONNX Runtime version requires editing exactly one place.
• Docs, install scripts, and CI all derive the version from that single source.
• A check (CI or pre-commit) fails if the versions diverge.