Per-shard merge scheduler that triggers compaction when file rotation occurs.

Each shard has its own Scheduler GenServer. Instead of polling every 30s with expensive File.ls calls that block the Shard GenServer, the scheduler is event-driven: the Shard notifies it on file rotation via notify_rotation/2.

Merge modes

The scheduler supports three merge modes per the spec (section 2E):

• Hot mode -- Event-driven, can trigger anytime. When the file count reaches min_files_for_merge after a rotation, the scheduler attempts a merge (subject to the node-level semaphore). This is the default mode.

• Bulk mode -- Only merges during a configurable time window (e.g. 02:00-04:00). Outside the window, rotations are noted but no merge is triggered. Inside the window, file count is checked.

• Age mode -- Like bulk mode but only merges files older than a configurable age threshold, within a time window.

Merge lifecycle

1. Shard rotates its active file and casts {:file_rotated, file_count}.
2. Scheduler checks file count against min_files_for_merge and mode.
3. If merge is needed, scheduler requests the node-level semaphore.
4. If semaphore is acquired, scheduler writes a merge manifest.
5. Scheduler selects non-active files for incremental merge.
6. Scheduler calls the shard's run_compaction via GenServer.call.
7. On completion, scheduler deletes the manifest and releases the semaphore.
8. On failure, scheduler logs the error, deletes the manifest, releases semaphore.

Configuration

Configuration is passed via the :merge key in the application env:

config :ferricstore, :merge,
  mode: :hot,
  min_files_for_merge: 2,
  max_files_per_merge: 10,
  merge_window: {2, 4},
  min_free_space_ratio: 0.1