feat(ppvm-vihaco): add the composite machine and VM (#174)

## Merge order

Part of the #168 split. This PR targets **#173**
(`david/42.2-circuit-component`) and sits at the top of the vihaco line:

1. RotXY / R gate (#170) → `main` ✅ merged
2. tableau expectation/reset (#172) → `main` ✅ merged
3. vihaco-circuit-isa (#169) → `main` (open; carries #170 + #172 via a
`main` merge)
4. native `.pyi` stubs (#171) → `main` (open; independent)
5. circuit component (#173) → #169
6. **composite machine + VM (this PR)** → #173
7. CLI + TUI (#166) → this PR

## Summary

Completes the `ppvm-vihaco` crate on top of the circuit component (#173)
— the composite machine that drives it plus the surrounding VM.

| File | What |
|------|------|
| `composite.rs` | the `PPVM` composite: vihaco machine (CPU + program
loader) driving the circuit component, measurement/trace observers,
backend selection from the device header, reset, single-instruction
stepping |
| `syntax.rs` | `.sst` header parse / lower (`PPVMHeader`,
`PPVMResolver`) |
| `bytecode.rs` | `.ssb` bytecode emit / load (magic-tagged) |
| `observable.rs` | observable-header parsing for the PauliSum backends
|
| `shots.rs` | multi-shot execution (serial + optional `rayon`) |
| `lib.rs` | full module wiring + `load`/`run`/`dump`/`compile`/`parse`
helpers + `PPVMModule` |
| `Cargo.toml` | adds `vihaco-cpu`, `log`, optional `rayon` feature |
| `tests/*.sst` + `sst_fixtures.rs` | 28 integration fixtures |

## `device_info` relocation resolves here

`composite.rs` no longer defines `PPVM_MAGIC` / `BackendKind` /
`PPVMDeviceInfo` — it imports them from `device_info` (introduced in
#173) and `pub use`-re-exports them, so the existing
`crate::composite::{…}` paths in `lib.rs` / `bytecode.rs` / `syntax.rs`
keep resolving with no other edits.

The machete-ignore added in #173 is dropped here — the full crate uses
`chumsky` / `vihaco-parser-core` directly.

## Testing

`cargo test -p ppvm-vihaco` — **111 unit + 28 integration** tests, green
with and without `--features rayon`. Full workspace builds; clippy +
machete clean.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

Cargo.lock

@@ -3,21 +3,25 @@ name = "ppvm-vihaco"
 version = "0.1.0"
 edition = "2024"
 
+[features]
+# Parallelism via rayon: shot-level (`run_shots`) and tableau-internal. Off by
+# default; enable with `features = ["rayon"]` (native only — wasm has no thread
+# support, so keeping it off by default keeps the default build wasm-safe).
+rayon = ["dep:rayon", "ppvm-tableau/rayon"]
+
 [dependencies]
 bitvec = "1.0.1"
 bnum = { version = "0.13.0", features = ["num-traits"] }
 chumsky = "0.10.0"
 eyre = "0.6.12"
+log = "0.4.29"
 num = "0.4.3"
-smallvec = "1.15.1"
-ppvm-pauli-sum = { version = "0.1.0", path = "../ppvm-pauli-sum" }
+rayon = { version = "1.10", optional = true }
 ppvm-tableau = { version = "0.1.0", path = "../ppvm-tableau" }
+smallvec = "1.15.1"
 vihaco = "0.1.1"
+vihaco-cpu = "0.1.1"
 vihaco-parser = "0.1.1"
 vihaco-parser-core = "0.1.1"
 vihaco-circuit-isa = { version = "0.1.0", path = "../vihaco-circuit-isa" }
-
-# chumsky and vihaco-parser-core are pulled in by the vihaco_parser::Parse
-# derive on BackendKind; machete can't see macro-expanded usage.
-[package.metadata.cargo-machete]
-ignored = ["chumsky", "vihaco-parser-core"]
+ppvm-pauli-sum = { version = "0.1.0", path = "../ppvm-pauli-sum" }