features.yaml config loading in controller

[ben-malbeclabs]

Add -features-config flag to the controller. Define config struct covering flex_algo.enabled (gates topology config and link admin-group tagging together) and community_stamping (granular per-tenant and per-device control). Restart required to pick up changes.

Milestone: RFC-18: Controller — Topology Config
RFC: #3288

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Files affected:

• controlplane/controller/cmd/controller/main.go
• controlplane/controller/internal/controller/ (new config struct)

Size: Small

Tests:

• Missing file handled gracefully on startup
• Malformed YAML errors on startup
• enabled and community_stamping fields parse correctly

Blockers: None (can be parallelized with CLI milestone)