Playlists: move a playlist or folder between folders

[cole-hackman]

Folder-drop and drag-between were listed as two gaps. They are one missing change kind. Playlists tree moves to done.

PlaylistReorder already writes djmdPlaylist.Seq, and its SQL puts the parent in the WHERE clause specifically so a reorder cannot move anything between folders. That was the right call — a reorder that restructured the tree would be a nasty surprise — and it means moving needs its own verb.

Two refusals, because djmdPlaylist enforces neither

• The destination must be a folder. Rekordbox nests under folders only; a playlist parented to a playlist is a shape nothing renders, so it would simply vanish from the sidebar.
• A folder cannot go inside itself or a descendant. This is the one that actually worried me: it does not error, it loses no data, it just detaches that entire subtree from the root. The playlists still exist in djmdPlaylist, and there is no path to them from the tree, ever. Silent and total.

Three smaller decisions

• The UI mirrors both refusals rather than letting the applier be the only guard. Duplication I would normally argue against — but without it the drop looks like it worked, and the rejection surfaces only when the user opens the review table, by which point they have made several more drops on a tree that was lying to them. A folder highlights only when the drop would actually be accepted.
• The ancestor walk goes upward from the destination, not downward from the dragged folder: a playlist tree is far wider than it is deep, so the upward walk is bounded by depth while the downward one visits the whole subtree. Both the Rust and TypeScript versions keep a seen set — a database that already contains a cycle must not hang the sync or the render, and the move is not what created that problem, so it reports "not a descendant" and lets the write through.
• old_parent_id rides on the change. Without it changes::undo blocks the inverse and the tree cannot be put back — and for a drag, the easiest edit in the app to make by accident, that is exactly where undo matters.

The rules live in lib/playlist-tree.ts rather than inside the drop handler, for the same reason reorder.ts exists: jsdom does not run drag events, so a rule that lives in a handler is a rule nothing tests.

The move is staged, not written — the tree redraws from master.db, so the row does not appear to move until Sync applies it. Honest behaviour for a change that has not happened yet.

Verification

cargo fmt --all -- --check          clean
cargo test --workspace              54 test binaries, 0 failures (changes +11)
cargo clippy --all-targets -D warn  clean
pnpm test                           807 tests, 70 files
pnpm typecheck / pnpm lint          clean
pnpm e2e                            59 passed

Matrix moves to 59 done / 21 partial / 14 missing / 2 blocked / 16 deferred; Library & browser is now 16 done / 1 partial, the remaining one being the track table's missing drag source.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6224e20 and a78ba01.

📒 Files selected for processing (13)

• apps/desktop/src-tauri/src/lib.rs
• apps/desktop/src-tauri/src/playlist_tools.rs
• apps/desktop/src/components/PlaylistPanel.tsx
• apps/desktop/src/ipc.ts
• apps/desktop/src/lib/playlist-tree.test.ts
• apps/desktop/src/lib/playlist-tree.ts
• crates/changes/src/applier.rs
• crates/changes/src/applier/playlists.rs
• crates/changes/src/lib.rs
• crates/changes/src/undo.rs
• docs/JOURNAL.md
• docs/STATUS.md
• docs/lexicon/PARITY.md

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