[Twopic2]

Adds direct video drag and drop from the Linux file manager, bypassing the Media Browser.

• Dropping onto Source imports the video and opens it for preview.
• Dropping onto Timeline places video with linked audio and supports undo.
• Adds hover feedback, import-error reporting, and regression coverage.
• Fixes VA-API H.264 fallback during open-GOP seeks and validates linked-selection parameters.
  Linux automatically uses X11/XWayland when available. Native Wayland-only sessions still require Import.
  Validation: 3,505 workspace tests passed. Formatting, Clippy, layering, asset attribution, and WebAssembly checks passed.

[FredApps]

This PR holds several unrelated changes in one commit, and they can't be reviewed or reverted separately. Could you split it into:

1. Linux file-manager drops (linux_window.rs, main.rs, lib.rs, monitor.rs, timeline.rs, timeline_drop_ui.rs, x11rb and the winit features in apps/filmcraft/Cargo.toml, crates/ui-egui/README.md, docs/control-protocol.md, docs/gaps.md).
2. The VA-API H.264 open-GOP seek fix (crates/platform/src/vaapi/*, crates/platform/README.md). It has nothing to do with drag and drop and needs its own hardware validation.
3. The timeline.select linked override and the Alt-box selection change (crates/engine/src/commands.rs:2593, tests.rs, timeline_marquee_ui.rs). The engine part also adds a rejection: linked that is not a boolean now returns an error, where main treats a non-bool as "follow links". Please say whether any caller passes a string or number there.
4. The MimeType= line in apps/filmcraft/src/dev_icon.rs. #1958 is the same fix and is open, so this hunk should be dropped here.

The ROADMAP.md edits (a new "Last updated" date and three progress-log lines, one per change above) will conflict with every other PR that touches that table, so each split PR should carry only its own line.

On the feature itself: the PR text says native Wayland-only sessions still need Import, and the drop path prefers X11 when DISPLAY is set and a connection succeeds. That changes the window backend for existing Wayland users who also have XWayland. Is the X11 preference acceptable for people who run Wayland on purpose, for example for fractional scaling? That needs a maintainer decision. The FILMCRAFT_WINDOW_BACKEND override is documented in crates/ui-egui/README.md, which helps.

I have not requested validation, because the split will change the heads.