FastPlay v0.4.0 - Lightweight Play Queue and Folder Playback

FastPlay 0.4.0 adds a lightweight, in-memory play queue: drop several files or
a folder, step through them with PageUp / PageDown, and have FastPlay
auto-advance to the next file when the current one reaches its natural end. It
stays true to the charter — PlaybackSession remains the single, concrete,
single-file coordinator, and the queue is owned by the event loop next to the
recent-files state. No playlists, no media library.

Highlights

• Added a lightweight play queue from multi-file and folder drops
• Added manual previous/next navigation with PageUp / PageDown
• Added auto-advance to the next queued file at the natural end of playback
• Preserved single-file behavior: a normal open never scans the parent folder
• Kept PlaybackSession a single-file coordinator; the queue lives in the event loop

New Features

Play queue

• Drop multiple files onto the window to build a multi-item queue
• Drop a folder to queue its supported media files (non-recursive)
• Entries are filtered to supported media (subtitles excluded), de-duplicated
  case-insensitively, and ordered naturally (Episode 2 before Episode 10)
• Opening a single file (CLI, file dialog, drag/drop, or Recent Files) creates a
  single-file queue and does not scan the parent folder

Manual navigation

• PageDown opens the next queued file; PageUp opens the previous one
• Navigation only acts when the queue has more than one item and never wraps at
  the first or last entry
• A brief status overlay reports the result (Next: …, Previous: …,
  No next file, No previous file, Open failed)

Auto-advance

• When a file in a multi-item queue reaches its natural end, FastPlay opens the
  next queued file automatically
• Auto-advance does not run for a single-file queue and does not wrap at the end
  of the queue (it shows End of queue instead)
• Looping, auto-replay, and in/out range stops are respected: auto-advance does
  not fire when you are intentionally looping or replaying a range

Architecture Notes

• PlayQueue is a pure, unit-tested data structure owned by the event loop;
  PlaybackSession holds no queue state
• All opens — manual and automatic — flow through the existing
  open_media -> session.open_at path, so resume, recent files, subtitles,
  metrics, and in/out reset behave identically to a manual open
• PlaybackSession exposes a single new, minimal, edge-triggered
  take_ended_signal() that fires once per natural end-of-file transition and is
  consumed exactly once, so auto-advance cannot fire every frame while ended
• Candidate/commit discipline: next/previous lookups never move the cursor; the
  cursor commits only after the target file opens, so a failed open leaves the
  queue position unchanged

Controls

Key	Action
PageDown	Next file in the play queue
PageUp	Previous file in the play queue

Validation

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets (140 passing)
• git diff --check
• Manual playback smoke:
  • drop multiple videos; let the first end; confirm the second opens
  • let the last queued video end; confirm it does not wrap
  • use PageUp / PageDown; confirm manual navigation works
  • open one normal file; let it end; confirm no parent-folder auto-advance
  • confirm recent files and resume still work
  • confirm loop / auto-replay / in-out range behavior is unaffected

Upgrade Notes

• Existing installs upgrade in place through the MSI major-upgrade path
• The play queue is in-memory and session-only; nothing new is written to disk
• No account, network, or cloud storage is required

Deferred (intentionally out of scope)

• Persistent playlist files, shuffle, repeat modes, recursive folder scanning
• A multi-select open dialog (multi-item queues are built via drag-and-drop)
• An in-window queue list / overlay, preloading, and gapless playback
• Media-library behavior