Execute semicolon-separated Programs across Rust, Python, and CLI

## Parent

- #7

## What to build

Move semicolon execution onto the accepted typed Engine and RFC-0003 ProgramResult without regressing compatibility `roll()` or `run()`.

## Frozen contract

- Strict `Engine.roll` accepts exactly one Instruction and returns `InstructionResult`; `Engine.run` accepts source text and returns `ProgramResult`.
- Top-level instructions are source ordered and share one request-wide RollNode/TraceNode pool, RNG, immutable environment, deadline/token, and budget.
- Each InstructionResult has a unique activation ID, frame-local index, byte span, canonical text, typed value, scalar projection, trace root, and nested executions.
- Repeat iterations, selected branches, and function bodies remain under their activating instruction instead of flattening into top-level history.
- Program comment is decoded text or null in strict v2; metadata records compatibility mode and replay descriptor.
- Any instruction failure atomically returns one structured exception/error envelope and no completed prefix.
- Module-level `roll`/`run` retain v1 dictionaries through adapters over the same Engine.

## Acceptance criteria

- [ ] `Engine.run("1d6; 2d6 # encounter")` returns two ordered typed instructions plus Program metadata.
- [ ] `Engine.roll` rejects separators and returns one typed InstructionResult.
- [ ] Rust, installed Python objects, `to_dict()`, and CLI JSON validate against RFC-0003 and agree on ordering and IDs.
- [ ] Nested indices, spans, trace roots, values, and read-only collections satisfy graph invariants.
- [ ] All instructions share one RFC-0002 ExecutionContext and counter set.
- [ ] Empty instructions and mid-Program failures return structured atomic errors.
- [ ] Compatibility `roll()` and `run()` retain legacy dictionaries through Engine adapters.

## Blocked by

- #7 human acceptance
- #4 human acceptance
- #5 human acceptance
- #39

Start with the typed Engine.run and Engine.roll entry points and read the RFC-0002 ExecutionContext and RFC-0003 ProgramResult contracts. Trace the Rust implementation, installed Python objects, to_dict(), and CLI JSON paths. Done means semicolon-separated programs, atomic errors, shared execution state, nested metadata, and compatibility adapters agree across all surfaces.

Written by the indexing model from the issue text.