feat(migrate): physical index/constraint attrs + auto schema-scope + DB-adoption fixes

[dmealing]

Summary

Enables non-destructive adoption of an existing Postgres database (verify --db / migrate --from-db reaching zero drift against a hand-built schema) and richer index modeling. All new physical-storage attributes are contributed by the db provider (spec/metamodel/db.json extends), not core.

Changes

db provider — physical attrs on core identity subtypes

• identity.secondary @orders — per-key sort direction (asc|desc) → DESC index keys
• identity.secondary @where — partial-index predicate → WHERE (<pred>)
• identity.reference @constraintName — FK constraint-name override (adopt an existing DB's FK naming without a rename)

Wired end-to-end: IndexDescriptor {orders?, where?}, buildSecondaryIndexes, renderCreateIndex (DESC + WHERE), readPgIndexes (indoption + pg_get_expr(indpred); expression-key indexes skipped), indexEquals (positional orders + normalized predicate); FkDescriptor honours @constraintName.

diff — auto schema scope (DiffArgs.scopeSchemas)
The diff manages only the schemas the expected/metadata side declares; a table in an undeclared schema belongs to another owner and is left untouched. Per-owner drift gates become clean with no manual config. Explicit set overrides; empty model preserves prior whole-DB behavior.

introspect — gen_random_uuid() as an expression default
parsePgDefault now treats any bare function-call default as an expression (mirrors the metadata-side EXPR_DEFAULT_PATTERNS), so a uuid-PK column round-trips without a spurious default diff.

fix(metadata) — allowedValues on an isArray attr validates each element, not the array as a whole (@orders is the first isArray+allowedValues attr).

Tests

New: index-partial-ordered, diff-schema-scope, expected-schema-fk-constraint-name, isArray-allowedValues cases. Updated: diff-schema-aware, identity-completeness + coverage gates. Regenerated the embedded metamodel, registry manifest, metamodel docs, and Python spec copies. metadata + migrate-ts suites green (2521 pass).

Cross-port note

This grows the canonical metamodel vocabulary (new db-provider attrs). The TS port and the Python spec copies are updated here; the Java / Kotlin / C# ports will need to reconcile their registry-conformance against the regenerated expected-registry.json (expected RED until they implement the new attrs), per the documented multi-port reconciliation flow.

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