Add :mode parameter to defslot with three behaviors:
- :stack (default) — accumulate all registrations, render in order
- :replace — each registration replaces previous entries
- :single-winner — first registration wins, rest ignored
Mode is set on first defslot call and frozen for subsequent calls
to prevent conflicting mode specifications from different plugins.
Store slot data as plist (:mode <keyword> :entries <list>) instead
of bare entries list.
Add 5 new tests covering mode-specific behavior. All 9 slot tests
pass. All 13 suites pass at 100%.
6cd045ff59