test(konsist): enforce that domain models are immutable

[simtop]

The last invariant in AGENTS.md §3 that had no rule behind it. All six domain models already comply, so this is a ratchet, not a fix — it stops the seventh from being written differently.

Two rules, because there are two ways to break it

A var property is the obvious one. The other is a val holding a mutable collection:

data class BeerPage(val items: MutableList<Beer>, ...)   // reads immutable; isn't

The reference is fixed, the contents are not. Whoever receives the model can call items.add(...) and change what the sender is still holding. That's the half that slips through review, which is why it's worth enforcing rather than trusting.

Array is banned for a second reason: identity equality gives a data class an equals that reports two identical models as different — breaking the same Compose recomposition comparison from the opposite direction.

Why this matters beyond tidiness

PagedListReducer folds pages into a list and hands it to Compose, which decides what to recompose by comparing old state to new. A model mutated in place is the same reference, so the comparison says "unchanged" and the UI silently keeps rendering stale data. The mappers and the pager pass the same instances around on the same assumption.

Verified against real violations, not assumed

Both halves were checked by introducing a violation into a real model in the primary constructor and confirming the build fails:

• data class BeerStyle(val id: String, var name: String) → fails
• data class BeerPage(val items: MutableList<Beer>, ...) → fails

That specifically confirms Konsist's properties() sees primary-constructor properties. Had it not, the rule would have passed vacuously on every data class here — that's where all of them declare their state, so the check would have been decorative.

Docs kept in step

AGENTS.md and the README rule table go from eight to nine in the same commit, and AGENTS.md §3 now states that every invariant it lists is mechanically enforced, with a note to add the rule alongside any new one.

make konsist · spotlessCheck · make lint — green.