Add OpenQASM 3 circuit input via from_qasm (#14)

[mk0dz]

Closes #14

What was built

from_qasm(qasm::String) -> GateCircuit in src/qasm.jl, plus the include +
export from_qasm in src/Stretto.jl.

Coverage (vs #24's 9 discrete gates):

• Discrete: id, x, y, z, h, s, sdg, t, tdg, sx, cx/cnot, cz, swap, ccx/toffoli, ccz
• Parametric: rx(θ), ry(θ), rz(θ), p/phase/u1(θ), cp/cphase(θ), crz(θ), u/u3(θ,φ,λ)
• Registers: qubit[N] q; (QASM3) and qreg q[N]; (QASM2); 0→1 based index conversion
• Headers (OPENQASM, include), // and /* */ comments, barrier, gphase ignored
• Unsupported (measure/reset/classical/control-flow/custom-gates/multi-register) → ArgumentError

Idiomatic to Stretto: parametric gates register a unique symbol + matrix in
EXTRA_GATES and emit GateOp(sym, qubits) — exactly how library.jl's
qft_circuit handles cp_gate. So circuit_unitary resolves them with no core
changes.

Security/quality note: angle expressions (pi/2, 3*pi/4, tau, -0.5) are
evaluated by a small hand-written recursive-descent arithmetic parser — no
eval, so untrusted QASM can never execute code. (Replaced an initial
whitelisted-eval version after a security lint flagged it; the parser is both
safer and cleaner for a library.)

Validation

8 @testitem blocks, 17 assertions, all pass, each checking correctness by
round-tripping through circuit_unitary (the issue's acceptance criterion):

• qubit count + 0→1 index conversion (both register syntaxes)
• discrete round-trip (bell) and Toffoli vs toffoli_circuit()
• parametric rx/ry/rz/p vs their matrix definitions; two-qubit cp vs diag(1,1,1,e^{iθ})
• angle arithmetic (3*pi/4 ≡ -pi/2 + 5*pi/4; tau ≡ 2*pi)
• comments/barriers ignored
• 5 error cases (measure, unknown gate, out-of-range index, 2 registers, no declaration)

Package loads cleanly (the change is additive: one new file + include/export),
so the existing suite cannot regress from it.

Files changed (PR contents)

 src/qasm.jl       (new: parser + 8 @testitem tests)
 src/Stretto.jl    (+2: include("qasm.jl"); export from_qasm)

AI Acknowledgement: I would like to thanks claude sonnet 4.6 to make me understand the codebase and structure of Stretto

[jack-champagne]

We have many quality submissions and it was a tough decision, but there is still so much work to do on this package and we encourage everyone who made a submission to continue contributing and making suggestions, PRs, and issues on this repo. Thank you all!