Fix con_id criteria and move-to-mark for workspaces in i3 RPC

[CyberShadow]

Summary

This PR fixes [con_id=X] criteria to match workspace nodes, bringing sway's behavior in line with i3. It also adds marks support for workspaces and fixes move container to mark behavior for split targets.

Problem

In i3, [con_id=X] works for all node types including workspaces. In sway, it only matches containers, breaking scripts that rely on targeting workspaces by their internal ID.

Reproduce on master:

# Get a workspace ID
swaymsg -t get_tree | jq '.. | objects | select(.type == "workspace") | {name, id}' | head -8

# Try to mark it (fails)
swaymsg '[con_id=<workspace_id>] mark test'
# Returns: "No matching node."

Use Case

A "move to parent" helper that reparents a container to be a sibling of its current parent:

# Move focused container up one level in the tree
[con_id=$grandparent] mark --replace _target
[con_id=$parent] focus
[con_id=$container] move container to mark _target
unmark _target
[con_id=$container] focus

This works in i3 but fails in sway when the grandparent is a workspace.

Changes

1. fix(criteria): allow con_id to match workspaces - Extend criteria_get_nodes() to iterate workspaces when con_id is specified

2. feat(workspace): add marks support to workspaces - Workspaces can now have marks like containers, enabling the full "move to mark" pattern

3. fix(move): match i3 behavior for move-to-mark on split targets - When moving to a mark on a workspace or split container, descend to the focused child and add as sibling (matching i3's con_move_to_target behavior)

Test Plan

# 1. Get workspace ID
WS_ID=$(swaymsg -t get_tree | jq '[.. | objects | select(.type == "workspace" and .name != "__i3_scratch")][0].id')

# 2. Test con_id matching (should succeed)
swaymsg "[con_id=$WS_ID] mark test_ws"

# 3. Verify mark was added
swaymsg -t get_tree | jq ".. | objects | select(.id == $WS_ID) | .marks"
# Should output: ["test_ws"]

# 4. Test move to workspace mark
# Create a split with nested container, then:
swaymsg "[con_id=$WS_ID] mark parent"
swaymsg "move container to mark parent"
# Container should become direct child of workspace

# 5. Test move to container mark (split target)
# Mark a split container, move a nested container to it
# Container should become sibling of focused child, not nested deeper

# 6. Cleanup
swaymsg "unmark test_ws; unmark parent"