Rewrite large object migration as a direct relation copy.

[ibrarahmad]

Based on #55.

Migration used to rewrite every large object through the LO API, which drops things the API cannot carry. It is now a direct copy of tuples between the native and lolor relations, in C (lolor.migrate_storage(), src/lolor_migrate.c). The layouts match by construction and are checked at run time.

What that fixes: comments were lost (now parked in lolor.pg_largeobject_description and put back); ownership was set with a raw UPDATE, so DROP ROLE, REASSIGN OWNED and DROP OWNED did not see migrated objects (now recorded in pg_shdepend); sparse objects were zero-filled (pages are copied as is); native objects are removed in batches through performMultipleDeletions().

Migration holds ShareRowExclusiveLock on both stores until commit, so a concurrent lo_write() cannot land between the copy and the delete and be lost. Locks are taken in a fixed order.

Both migrations also refuse while any other session is connected to the database, for the same reason enable() and disable() do in #55. The concurrent-write test in t/010 is now a refusal test, and the lock is checked from the migrating session itself.

Also: migrate_to_native() no longer needs lolor enabled, so DROP EXTENSION works in the disabled state; migration refuses on security labels and on objects that refer to a dropped role, with a clear message instead of "role N was concurrently dropped"; OID conflicts list the OIDs; lolor.digest() and lolor.native_lo_oids() with a peer_oids check for cross-node collisions; lo_unlink() removes a parked comment so a reused OID cannot inherit it. The README no longer suggests running the migration through spock.replicate_ddl(), since that runs it inside apply workers.

1.3.0 is unreleased, so the SQL goes into lolor--1.2.2--1.3.0.sql.

Tests: t/009_migration.pl and t/010_migration_locking.pl, plus regression coverage for sparse objects, comments, pg_shdepend, orphans and the helpers.

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[codacy-production]

Up to standards ✅

🟢 Issues 16 high · 5 medium

Results:
21 new issues

Category	Results
Compatibility	16 high (4 false positives)
Complexity	5 medium

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🟢 Metrics 67 complexity · 2 duplication

Metric	Results
Complexity	67
Duplication	2

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