refactor: split org-drill-merge-buffers into named phases

merge-buffers was 89 lines of dense cross-buffer marker work mixing
hash-table population, source-buffer iteration with embedded
property reads, scheduling-data writes, and cleanup of unmatched
DEST entries.

Extracted four helpers:

- org-drill--build-dest-id-table: scan DEST and populate the id→
  marker table.
- org-drill--copy-scheduling-to-marker: read the current entry's
  scheduling state and write it at MARKER (skipping never-rated
  items via total-repeats=0 guard).
- org-drill--migrate-from-source: walk SRC, dispatch to the copy
  helper or org-drill-copy-entry-to-other-buffer for new items.
- org-drill--strip-unmatched-dest-entries: clean up DEST entries
  that have no SRC match.

merge-buffers itself drops from 89 lines to 19 and reads as a
sequence of named phases.

0 files changed, 0 insertions, 0 deletions