feat(w10a): engine TextPairWithMetadata extra_columns + kei-crossdomain re-migrated
EdgeKeyKind::TextPairWithMetadata extended with:
- from_col / to_col (custom column names, default src_path/dst_path)
- extra_columns: &[(name, FieldKind)] for domain-specific edge metadata
kei-crossdomain fully re-migrated via engine:
- edge_table: Some('cross_edges') + TextPairWithMetadata variant with
from_col='from_uri', to_col='to_uri', has_id/has_weight/has_created_at,
extra_columns=[evidence, metadata]
- Custom edges DDL dropped from custom_migrations (engine owns it now)
- edges.rs query_edges SELECT uses edge_id (engine-emitted PK)
Tests: 42/42 kei-entity-store (+2), 5/5 kei-crossdomain preserved.
Sister crates (task/chat/content/social/sage) no regression.
Closes HANDOFF-WAKE deferred item #2.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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