intent reconciler: NodeNetplanConfig write conflicts between InterfaceConfig and Intent controllers

[MaxRink]

Problem

Both the InterfaceConfig controller and the Intent reconciler write to the same per-node NodeNetplanConfig objects. Since both controllers reconcile independently, the last writer wins — meaning one controller can silently overwrite changes made by the other.

Impact

CRITICAL — Potential silent data loss during reconciliation when both controllers are active on the same cluster.

Affected Files

• controllers/platform/interfaceconfig_controller.go
• controllers/intent/intent_controller.go

Suggested Fix

Either:

1. Partition NodeNetplanConfig ownership (e.g., separate objects per controller), or
2. Use Server-Side Apply with distinct field managers, or
3. Make Intent the sole writer and have InterfaceConfig feed into Intent

Found During

Review Round 3 of PR #249 — tracked separately since it's a pre-existing design issue in the intent-based-crds feature, not introduced by PR #249.