Add A63 control-plane regression example

[zzw4257]

Summary

Adds examples/basic/A63_control_plane_regression, a compact example-first runtime regression entry for the control-plane foundation work.

The goal is to give reviewers and CI one high-value example to run after changes to Routing, BGP intent metadata, FRR rendering, ExaBGP service binding, or MPLS readiness, without adding a large scatter of separate default examples. The example uses the A63 number so it does not collide with lower-numbered basic examples or IPv6 work that may use A14.

What A63 Covers

• BIRD route-server baseline on IX100 with AS150/AS151 route-server clients.
• Mixed BIRD/FRR transit AS2 using shared OSPF, iBGP, and eBGP intent.
• FRR route-reflector AS3 with bgp cluster-id, route-reflector-client, and route propagation to an FRR RR client.
• ExaBGP as ExaBgpService + Binding only, peering with an FRR router in AS4 and announcing 198.51.100.0/24.
• MPLS/LDP readiness in AS20 with generated /mpls_ifaces.txt and FRR LDP config checks.

Design Notes

• This PR does not change core Routing, FRR, ExaBGP, or MPLS semantics.
• ExaBGP remains a service speaker, not a router backend.
• FRR route server is not introduced; A63 keeps the existing BIRD route-server baseline visible.
• MPLS dataplane is intentionally host-gated. The default A63 runtime validates config/readiness and records host capability instead of pretending GitHub hosted runners always provide MPLS kernel support.
• A63 focuses on control-plane route state and daemon behavior. The route-server probe checks learned routes rather than cross-host dataplane reachability, because the regression target is route-server control-plane behavior.

CI Integration

Adds A63 to the existing PR workflow as a single default basic example:

• static py-compile list
• manifest clean validation
• compile matrix
• runtime matrix
• workflow_dispatch selected-example options

A63 is marked requires_mpls: false because the full example must still run on hosted runners; MPLS kernel-dependent dataplane validation remains out of the default path.

Local Verification

Ran from pr/a14-control-plane-regression after renaming the example to A63:

git diff --check
python3 -m py_compile \
  seedemu/testing/base.py \
  seedemu/testing/internet.py \
  seedemu/testing/blockchain.py \
  seedemu/testing/satellite.py \
  seedemu/testing/registry.py \
  seedemu/testing/runtime.py \
  seedemu/testing/cli.py \
  examples/basic/A63_control_plane_regression/control_plane_regression.py \
  examples/basic/A63_control_plane_regression/test_runtime.py
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 /tmp/seedemu-a14-venv/bin/python -m pytest -q test_frr_exabgp_foundation.py
/tmp/seedemu-a14-venv/bin/python -m seedemu.testing.cli clean examples/basic/A63_control_plane_regression/example.yaml
COMPOSE_PROJECT_NAME=seedemu-a63-control-plane \
DOCKER_CONFIG=/tmp/seedemu-docker-config-a14 \
/tmp/seedemu-a14-venv/bin/python -m seedemu.testing.cli all \
  examples/basic/A63_control_plane_regression/example.yaml \
  --artifact-dir ci-artifacts/a63-all-final

Results:

• git diff --check: passed.
• CI-style py-compile list including A63: passed.
• test_frr_exabgp_foundation.py: 11 passed.
• A63 full lifecycle: passed build, up, readiness, probes, custom runtime validation, and down.
• A63 generated compose was checked to ensure no unexpected IPv6 fields (enable_ipv6, ipv6_address, or IPv6 IPAM) appear.

Environment Note

The local WSL Docker config had a Windows credential helper entry that makes Python subprocess Docker builds fail with docker-credential-desktop.exe: exec format error. The successful lifecycle run used an isolated temporary Docker config without that host-specific credential helper:

DOCKER_CONFIG=/tmp/seedemu-docker-config-a14

This does not affect CI and is not part of the repository change.