Re-evaluate "default_orientation auto" when workspaces change outputs

[mil-ad]

When default_orientation auto is set, workspace layout (horizontal vs vertical) is determined by output dimensions at creation time but never re-evaluated. This means a workspace created on a landscape display keeps L_HORIZ even after moving to a portrait display, which defeats the
purpose of auto orientation in multi-monitor setups.

This patch adds workspace_reorient_auto() which re-evaluates the layout based on the current output's aspect ratio. It is called when:

• A workspace is moved to a different output (move workspace to output)
• An output is disconnected and workspaces evacuate
• An output is reconnected and workspaces restore back
• An output's mode or transform changes (e.g. rotation)

The function is conservative and only acts when:

• default_orientation is auto (explicit horizontal/vertical is never touched)
• The workspace has at most 1 tiling child (existing multi-window layouts are preserved)
• The workspace is not tabbed or stacked