- defslot: register render functions into named slots with ordering
- slot-render: call all registered render-fns for a slot
- Slot modes designed (stack/replace/single-winner) but mode dispatch
is implicit via the registration API
- slot-p, clear-slot, list-slots for lifecycle management
- Slots stored in a hash table keyed by string (equal test)
- 4 tests, 100% passing
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