feat(canonical-migration): --remap-existing to re-evaluate already-migrated edges (#45)

[jphein]

Why

The first-migration UPDATE in scripts/canonical_migration.py is one-shot by design: its AND NOT (properties ? 'raw_relation_type') guard makes a second run a no-op (every migrated edge already carries raw_relation_type). That's correct for the original rollout but blocks the #45 use case — after the predicate normalizer (mempalace.kg_predicate_norm, improved in mempalace#336) is upgraded, the bulk of the other bucket should be re-evaluated, and those edges are exactly the already-migrated ones a naive re-run skips.

On the production graph, 55% of 1.92M RELATION edges sit in other, each retaining its original predicate in raw_relation_type. The improved normalizer reclaims ~half of them — but only if the migration re-reads the original predicate and is allowed to overwrite an existing canonical.

What

--remap-existing (default OFF) switches both halves of the migration:

• Frequency read uses coalesce(r.raw_relation_type, r.relation_type) so the mapper re-evaluates the original predicate, not the prior (possibly other) canonical.
• Apply UPDATE keys the join on the same coalesced original, drops the first-migration guard, preserves the original (coalesce keeps an existing raw_relation_type, only backfilling it on never-migrated edges), and adds relation_type IS DISTINCT FROM m.canonical so only edges whose canonical actually changes are touched (no MVCC churn on no-ops).

Default behavior is byte-for-byte unchanged without the flag. Same backup + paused-worker gating applies (the --apply + --i-have-a-backup guards are untouched).

# dry-run the re-map (read-only via /cypher)
python scripts/canonical_migration.py --remap-existing
# apply (host-side, backup in hand, worker paused)
python scripts/canonical_migration.py --remap-existing --apply --dsn "$MEMPALACE_POSTGRES_DSN" --i-have-a-backup

Prerequisite chain

This is the mechanism for the #45 prod re-map. mempalace#336 (merged) improved the normalizer; this PR lets the existing graph reflect it without an LLM re-call (pure deterministic relabel of raw_relation_type). The actual prod run remains gated on a backup + explicit approval.

Tests

6 new tests in TestRemapExisting: remap cypher reads the original, apply drops the guard + keys on the coalesced original + uses IS DISTINCT FROM, default keeps the guard, plus a back-compat getattr(..., False) case for callers built before #45. 18/18 pass; ruff clean; module docstring updated with the new section + exact SQL.

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[gemini-code-assist]

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