Add rename handling spec and roadmap entry

docs/spec-details/Rename+Move-Handling.md

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+# Marlin — Rename & Move Handling
+
+**Integration Specification · v0.1 (2025-05-19)**
+
+---
+
+## 0 · Scope
+
+This document outlines how Marlin should respond when files or folders are renamed or moved. It extends the watcher life‑cycle design (DP‑003) so that metadata remains consistent without requiring a full re‑scan.
+
+## 1 · Background
+
+The current watcher maps any `notify::EventKind::Modify(_)` – including renames – to the generic `EventPriority::Modify` and merely logs the event:
+
+```
+415  let prio = match event.kind {
+416      EventKind::Create(_) => EventPriority::Create,
+417      EventKind::Remove(_) => EventPriority::Delete,
+418      EventKind::Modify(_) => EventPriority::Modify,
+419      EventKind::Access(_) => EventPriority::Access,
+420      _ => EventPriority::Modify,
+421  };
+...
+455  for event_item in &evts_to_process {
+456      info!("Processing event (DB available): {:?} for path {:?}",
+457            event_item.kind, event_item.path);
+458  }
+```
+
+No database update occurs, so renamed files keep their old `path` in the `files` table. The schema does have a trigger to propagate `path` updates to the FTS index:
+
+```
+72  -- When a file’s path changes
+73  DROP TRIGGER IF EXISTS files_fts_au_file;
+74  CREATE TRIGGER files_fts_au_file
+75  AFTER UPDATE OF path ON files
+76  BEGIN
+77      UPDATE files_fts
+78         SET path = NEW.path
+79       WHERE rowid = NEW.id;
+80  END;
+```
+
+## 2 · Requirements
+
+1. **Detect old and new paths** from `Rename` events provided by the `notify` crate.
+2. **Update the `files` table** with the new absolute path when the target remains inside a scanned root.
+3. **Mark as removed** if the new location is outside all configured roots.
+4. **Batch updates** to avoid excessive writes during large folder moves.
+5. **Integration tests** exercising rename and move scenarios across platforms.
+
+## 3 · Implementation Sketch
+
+* Extend `ProcessedEvent` to carry `old_path` and `new_path` for `Rename` events.
+* Upon flushing events, call `db::mark_dirty` for the affected row, then update the `files.path` column. The existing trigger keeps `files_fts` in sync.
+* For directory renames, update child paths with a single SQL `UPDATE ... WHERE path LIKE 'old/%'` inside a transaction.
+* Emit `Create` and `Remove` events for files crossing watch boundaries so `scan --dirty` can prune or index them accordingly.
+
+## 4 · Edge Cases
+
+* **Atomic cross-filesystem moves** may surface as `Remove` + `Create`; both should be handled.
+* **Concurrent modifications** while moving should result in the newer metadata winning when `scan --dirty` runs.
+
+## 5 · Future Work
+
+Large scale refactors (e.g. moving an entire project) may benefit from a high‑level command that updates tags and links en masse. That is outside the scope of this spec but enabled by accurate rename tracking.
+
+---
+
+*End of document*
+