Update plan: use rsync --itemize-changes for change detection, no dir walk

PLAN.md

@@ -20,7 +20,7 @@ backup storage grow proportionally to what actually changed.
 1. **Clear PEND** — remove all contents
 2. **rsync PREV → PEND** — seed locally (fast)
 3. **rsync SOURCE → PEND** — apply remote changes (only diffs travel over the wire)
-4. **Generate delta** — diff PREV vs PEND, produce per-file deltas + manifest
+4. **Generate delta** — parse rsync itemize output to get change list, produce per-file deltas + manifest
 5. **Commit delta** — write to DELTAS atomically
 6. **Promote PEND → PREV** — swap working area to become new base
 
@@ -47,6 +47,22 @@ State.json is a recoverable cache — can be reconstructed by scanning DELTAS.
 - `DELTAS/N/` exists but `last_complete` is N-1 → finish promotion, update state
 - state.json missing → reconstruct from directory scan
 
+## Change Detection
+
+No directory walk needed. rsync SOURCE→PEND is run with `--itemize-changes`, producing a
+machine-readable list of exactly what changed. Output is captured (not streamed) and parsed:
+
+| rsync prefix | Meaning  |
+|-------------|----------|
+| `>f+++++++++` | New file |
+| `>f.st......` | Modified file (any combination of change flags) |
+| `*deleting`  | Deleted file |
+| `cd+++++++++` | New directory (ignored for delta purposes) |
+
+Lines starting with `>f` or `*deleting` are extracted. The path is the remainder after the
+11-character itemize code + space. This becomes the change list fed directly into delta generation
+— no separate directory walk required.
+
 ## Delta Format
 
 Pluggable backend interface with two operations: