Add ping-pong loop mode

[emlcpfx]

Summary

Loop was on/off. This makes it a tri-state — Play Once / Loop / Ping-Pong — where ping-pong reverses direction at each end of the range instead of jumping back to the start.

What's included

• playback/loopmode.py - the frame-advance rule as a pure function, so the stepping behaviour (including the bounce at each end) is unit-testable with no reader, timer, or Qt. Preserves the existing off/loop semantics exactly.
• playback/player.py - SequencePlayer tracks a playback direction and steps through loopmode.advance(); a scrub resets to forward. set_loop_mode() added to SequencePlayer, MoviePlayer and the MediaPlayer facade; set_loop(bool) is kept and maps onto it.
• constants/__init__.py - LOOP_MODES.
• widgets/__init__.py - a "Loop Mode" submenu under Playback. loop_mode is the single source of truth and syncs the menu action, the submenu, and the timeline loop button. The mode (including ping-pong) now survives a media load.

Known limitation (please read)

Ping-pong reverses for image sequences only. FrameDeck's movie playback is time-driven and audio-synchronized — packets decode forward into a queue against a monotonic clock — so it cannot run backwards without backward decoding and breaking A/V sync. Movies therefore treat ping-pong as a normal loop, documented on MoviePlayer.set_loop_mode.

(reviewapp gets ping-pong for free because it frame-steps with a RAM cache and has no audio at all. Making it work for FrameDeck movies would mean a frame-stepping playback mode — a much larger change, happy to do it separately if you want it.)

Test evidence

$ pytest tests/test_loopmode.py
.........                                                                 [100%]
9 passed in 0.02s

Covers preserved off/loop behaviour, the bounce at both ends, a full ping-pong cycle (1-2-3-4-3-2-1-2...), no endpoint repeated on a bounce, and degenerate single-frame/empty ranges.

Verification notes

• Advance logic is fully unit-tested.
• Ping-pong on a real EXR sequence in the UI is render-verify pending.

Dependencies

Branched on the test harness (#7).

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