kvstore: a bag of changes to owners, singletons, and indexes

[nrc]

Makes the following changes (each change in separate commits):

• owners are declared as part of the schema rather than set on first use
• mutating functions leave the store consistent if the user function panics
• an optimisation to clear a whole index if possible
• convenience function for specifying an owner just once rather than on every operation
• return the base key from index-driven operations (kv: index method returns should include base table key #227)
• store singletons as fields rather than in a map and tie them to a particular store, rather than being 'floating'

closes #227
cc #217

AI declaration: I used Claude for some very mechanical changes. I have reviewed those changes.