ensure verifier checks characteristic type is extended from BaseCharacteristic

[matt-beanland]

When diffo_example migrated from 0.2.x to 0.3 the shape of the characteristic dsl had not changed by the type required subtly changed from a CharacteristicValue (pretty open) to a Characteristic (or actually something extended from BaseCharacteristic).

A verifier here that ensures characteristic type is extended from BaseCharacteristic would have made buggy code not compile with a good error message which would have been helpful here.

[matt-beanland]

After fixing this we found that the 0.3.0 release is borked, since the assigner was implementing a characteristicValue instead of a characteristic.

[matt-beanland]

Added Info.characteristic?/1 and updated VerifyCharacteristics and VerifyFeatures to emit a compile-time DslError when a value_type module doesn't extend BaseCharacteristic. Added two tests covering both contexts.

The verifier immediately caught a real bug

ShelfInstance had characteristic :slots, AssignableValue — AssignableValue is a TypedStruct (a value payload), not a characteristic resource. The fix was not simply swapping in AssignableCharacteristic either — :slots belongs in pools do, not characteristics do.

ShelfInstance refactored to match CardInstance

Moved :slots from characteristics do to pools do / pool :slots, :slot
Added Pool.update_pools to the define action
Switched Assigner.assign/4 (explicit thing) to assign/3 (reads thing from pool declaration)
AssignableCharacteristic — free calculation

Added a thing attribute (set by create_pools at build time) and a FreeValues calculation that returns (last - first + 1) - count(assigned) as a plain integer, loaded by default.

Net result: ShelfInstance now correctly mirrors CardInstance, the verifier prevents this class of bug at compile time, and AssignableCharacteristic surfaces free capacity directly.