Default HybridEngine to the Rust PHIR interpreter

[ciaranra]

Default HybridEngine's classical interpreter to the Rust implementation (RustPhirClassicalInterpreter), and strengthen the Rust↔Python parity coverage that backs the switch.

Change

_make_classical_interpreter(None) now returns the Rust interpreter instead of the Python one. The cinterp argument still lets callers choose: cinterp="python" for the pure-Python PhirClassicalInterpreter, cinterp="rust" (or the default) for the Rust-backed one, or a concrete instance. Rust is a core dependency (pecos_rslib always ships), so the default path is always available; no fallback added.

Motivation: the Rust interpreter is bit-for-bit equivalent to Python (parity suite) and materially faster.

Verification

• Default resolves to RustPhirClassicalInterpreter.
• quantum-pecos suite: 784 passed, slr_tests: 435 passed (full SLR→PHIR→HybridEngine path) with the Rust default — no feature gaps.
• Parity coverage strengthened (second commit): fuzz iterations 200→2000 (classical) / 200→1000 (quantum); classical fuzz now nests expressions and includes unary ~; added the example1 WASM program; added a QASM→PHIR generated-differential over the curated converter fixtures. ~3,200 direct Rust≡Python comparisons total, all green, in <3s.

Notes

• Users can opt back to Python via cinterp="python".
• This is a default-behavior change affecting all HybridEngine users.

[ciaranra]

Consolidated into #347 (the default-to-Rust switch + parity strengthening now live there); avoiding a separate PR.