GeezerVison

Stacked on #413.

## Description
Building a state machine takes about 25 SMT checks for a small class,
and each check creates its own Z3 context. The machine only depends on
specifications, which are all registered in the first pass, so
`TypeChecker` now builds it on the first state error and reuses it for
every later error in the same checker. Finding the receiver's current
states still runs at each error.

Other options considered:
- **Eagerly, after the first pass, for every class with `@StateSet`:**
correct programs would pay the SMT cost too. Only worth it once
something needs machines without an error, e.g. the fsm diagram.
- **Once per run, cached in `Context`:** `Context` would depend on
`state_machine`, which depends on `context` (a package cycle). The other
way is a static cache that tracks when specifications change. Neither
seemed worth it for the extra hits across classes.

## Cost
Extra time of `TestExamples` on the 269 cases that exist on `main`
(summed per case, one run each; `main` takes 80.3 s):
- #413, without the cache: +29.5 s
- with this PR: +10.6 s, spread over the 54 files with state errors

## Type of change
- [x] Code refactoring

## Checklist
- [x] `mvn test` passes locally

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <[email protected]>

## Description
#412, #413 and #414 were merged into their stacked base branches instead
of `main`: once #411 was merged, their bases were never retargeted.
Their code is only on `feat/state-hints-unknown`.

This PR applies the three reviewed commits on top of `main`, unchanged:
- Explain dead ends in failing state transitions (#412)
- List the calls that reach an allowed state when the state is unknown
(#413)
- Build each class's state machine once per checker (#414)

The hint and state machine files are identical to
`feat/state-hints-unknown`. Rebase-merging keeps the three commits
separate.

## Checklist
- [x] `mvn test` passes locally on top of the current `main` (#408,
#409)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>