[feat] Placement binding directory with cross-node replication

[St4NNi]

Adds the placement binding layer that maps a structured id's handle to its scope, document class, and strategy, and replicates those bindings across realm nodes.

• PlacementBinding record and a fail-closed BindingDirectory resolver: a handle resolves locally to (scope, document_class, strategy_id), and an unknown or conflicted handle fails closed rather than resolving to a wrong scope.
• Bindings are stored in the realm config document and replicated to every realm node through a new append-only RealmConfigPlacementBindingAppended admin operation.
• Deterministic conflict detection through the admin-document reducer: two nodes binding the same handle to different scopes converge to Conflicted on every node (both values retained, no winner is ever chosen), while same-tuple appends collapse to Bound. Binding provenance is excluded from the conflict identity so re-announces of the same tuple never spuriously conflict.
• Bindings are append-only and immutable; strategy removal is blocked while a binding still references the strategy.

Part of #403.

[St4NNi]

Superseded by: #444