Accumulate core-crate additions: capture requests, focus/workspace fixes, test coverage

Bundles several related changes to crates/core built up over this
session rather than committed incrementally:
- WindowManager::request_capture_workspace/drain_capture_requests (new
  manager/capture.rs) - backend-agnostic queuing for an off-screen
  workspace render, see the wayland-side commit for why this exists.
- focus_window now switches workspace as a side effect when the target
  isn't on the current one, matching Hyprland's focuswindow convention
  (manager/focus.rs).
- Assorted window/rules/theme field additions and their test coverage.

Left less granular than the repo's usual one-purpose-per-commit
convention deliberately: these accumulated across a long session
without being committed as they landed, and are too entangled
line-by-line to safely split apart now without risking mis-attributing
changes to the wrong commit message.

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