reconciler: convergence loops

For Go code, follow the repository Go anti-pattern rules.

Purpose

This package schedules convergence work. It owns no VM, image, snapshot, or traffic state.

Type	Responsibility
`VirtualMachineReconciler`	Runs VM reconciliation passes.
`NewSleepTransferReconciler`	Runs sleep staging, compression, upload, and eviction in a separate eight-worker pool.
`NewSleepRestoreReconciler`	Reconciles sleeping VMs and restores remote artifacts in a separate eight-worker pool.
`ImageReconciler`	Caches images, prunes unused images, kernels, and staged snapshots.
`MigrationReconciler`	Advances each active destination migration.
`GuestChecker`	Pings running guests every minute, so a silent guest counts as unavailable.
`passScheduler`	Runs a pass at startup, on an interval, and on request.

Each operation has its own timeout.
VM operations share a fixed number of slots across passes. A pass starts operations and does not wait for them, so one slow VM does not delay the next pass. A pass skips a VM whose operation still runs.
Sleep uploads and restores each use a separate pool of eight workers with the same timeout, backoff, wake, and shutdown behavior. They do not occupy normal VM reconciliation workers or wait for each other's pool.
Errors are logged. A later pass retries safe operations.
A failing VM waits twice as long after each failure, up to 1 minute. Wake clears the wait.
Errors after cancellation are suppressed.