* manifest: run leaf/tag repair on interrupted crosslink sweep

crosslinkSweep skipped the repairLeafTable/repairTagPropagation pass whenever linkCandidatesInOrder returned an error, including a context cancellation. After #89's batched sweep, batches commit as they go, so a sweep cancelled after committing at least one batch left the plink edges and tagxref origins those batches wrote without their leaf/tag consequences -- stale until a later sweep linked something, and a later sweep whose remaining candidates all link zero artifacts skips the same gate and leaves the window open longer than one sweep.

Gate the repair on linked > 0 rather than on sweep success and return the sweep's own error after repairing, so an interrupted-but-committed sweep still recovers derived leaf/tag state on the way out. The repair runs in its own context-free transaction, so a cancelled ctx interrupts the candidate sweep but not the recovery, preserving the batched commit/rollback consistency guarantees.

Closes #143.

* manifest: join repair error with sweep error on interrupted sweep

When both the sweep is interrupted and the deferred leaf/tag repair pass fails, the repair error was dropped entirely -- only the sweep error was returned. A real repair failure (disk full, corruption, constraint violation) during a cancelled sweep would then persist the exact stale leaf/tag window this fix closes with no signal.

Join both with errors.Join so the repair failure is surfaced while errors.Is(err, context.Canceled) still holds for the three callers (clone.go, session.go, handler.go), all of which wrap with %w and none of which use exact equality. check-in: 7bc832f339 user: [email protected] tags: trunk