sdk: regenerate device fixtures and add legacy/future-version variants

[elitegreg]

Part of the forward-compatible Device interfaces refactor.
Design: https://gist.github.com/elitegreg/e1d27c97034656a980fa8d7628ae16c5

Depends on #3660 (SDK readers).

Scope

Update the fixture generator and add two new fixtures that exercise the SDK fallback and forward-compat-skip paths.

Tasks

• sdk/serviceability/testdata/fixtures/generate-fixtures/src/main.rs:
  • Update generate_device to populate new_interfaces with two new-format entries (one Loopback Vpnv4 with a FlexAlgoNodeSegment, one Physical with user_tunnel_endpoint=true). Leave legacy interfaces: Vec::new() since the serializer rebuilds it.
  • Update meta.fields in the JSON sidecar to reflect the new on-disk shape.
• Add device_legacy.bin / device_legacy.json: hand-serialized device with only the legacy vec populated and no trailing bytes. Exercises the SDK legacy-fallback path.
• Add device_future_version.bin / device_future_version.json: like device.bin but with one new-vec element doctored to version=5 and size += N plus N trailing junk bytes after the known fields. Exercises the SDK skip-to-end path.
• Run make generate-fixtures.

Tests

• Go, Python, TS SDK tests load device_legacy.bin (fallback path) and device_future_version.bin (skip path).