M3.12: Source playback and monitor range editing

Play source video and audio, drag marked ranges as video, audio, or a linked pair, and route Source commands to clip marks and markers. Add shared draggable In/Out handles and range movement, preserving scrubber priority and explicit-only Program marks.

Include original code-drawn controls, documentation, attribution, and regression coverage for playback, placement, routing, undo, cancellation, and hostile parameters.

Assisted-by: Codex

ATTRIBUTION.md

@@ -99,6 +99,7 @@ is traced or derived from Adobe artwork.
 | Asset | Where |
 |---|---|
 | UI icons (tools, transport, panels, header) | `crates/ui-egui/src/icons.rs`: vector paths on a 16×16 grid |
+| Source and Program range braces and trim hover cues | `crates/ui-egui/src/panels/monitor.rs` and `source_range.rs`: original generic vector shapes by FilmCraft contributors, MIT OR Apache-2.0; no third-party artwork |
 | Align / distribute / paragraph-alignment glyphs in the graphics panel | `crates/ui-egui/src/panels/graphics.rs` (`align_glyph`): bars and lines drawn with the egui painter |
 | Demo footage (ocean sunset, aurora, city night, dunes, forest, plasma), bars and tone, counting leader, colour matte | `crates/media/src/generators.rs` |
 | Built-in graphics templates (Lower Third – Slab / Rule, Ticker – Crawl, Title – Centered / Boxed, End Credits – Roll, Callout – Pointer / Tag) and their placeholder text | `crates/project/src/gtemplate.rs` (`builtin_templates`): original designs defined in code with the bundled Inter font; thumbnails are rendered at runtime, no template, image or font files |