## PR-12 — VM test harness: stepper, trace, and property tests

**Why:** Industrial quality means test tooling, not just “it runs”.

### Scope

* Add `VmRunner` test harness:

    * step limit
    * deterministic trace of stack deltas
    * snapshot of locals
* Add property tests (lightweight):

    * stack never underflows in verified programs
    * verified programs never jump out of bounds

### Acceptance

* Debugging is fast, and regressions are caught.

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## PR-13 — Optional: Refactor Value representation (tagged slots) for clarity

**Why:** If current `Value` representation is the source of complexity/bugs, refactor now.

### Scope (only if needed)

* Make `Slot` explicit:

    * `Slot::I32`, `Slot::I64`, `Slot::U32`, `Slot::Bool`, `Slot::ConstId`, `Slot::GateId`, `Slot::Unit`
* Multi-slot types become sequences of slots.

### Acceptance

* Simpler, more verifiable runtime.

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## Definition of Done (DoD) for PBS v0 “minimum executable”

A single canonical cartridge runs end-to-end:

* `let` declarations (locals)
* arithmetic (+, -, *, /, %, comparisons)
* `if/else` control flow
* `when` expression (if present in lowering)
* function calls with params + returns (including `void`)
* multiple return slots (flattened structs / hardware value types)
* host syscalls (e.g., `GfxClear565`, `InputPadSnapshot`, `InputTouchSnapshot`)
* deterministic traps (OOB bounded, invalid local, invalid call target, stack underflow)