Fix qasm3 experimental exporter delay units for ms and ps

[gkneighb]

Summary

Fixes two independent bugs in qiskit.qasm3.dumps_experimental (and dump_experimental) where Delay instructions were serialised with the wrong unit or numeric value.

Fixes #16097.

What was wrong

1. ms was emitted as us — crates/qasm3/src/ast.rs's Display impl for DurationUnit mapped Millisecond to "us" (copy-paste from the previous arm). The emitted duration was 1000x smaller than intended.

2. ps → ns conversion was inverted — crates/qasm3/src/exporter.rs multiplied by 1000 instead of dividing. Since 1 ps = 0.001 ns, Delay(1, 'ps') was emitted as delay[1000ns], a factor of 10^6 too large.

OpenQASM 3 has no ps literal, so the exporter must convert to ns — that part is correct; only the conversion factor was wrong.

The non-experimental Python exporter (Exporter.dumps) was unaffected and already covered by TestCircuitQASM3.test_delay_statement.

Reproduction (from the issue)

from qiskit import QuantumCircuit, qasm3
import warnings

def emit(dur, unit):
    qc = QuantumCircuit(1)
    qc.delay(dur, 0, unit=unit)
    with warnings.catch_warnings():
        warnings.simplefilter("ignore")
        out = qasm3.dumps_experimental(qc)
    line = next(l for l in out.splitlines() if "delay" in l)
    print(f"Delay({dur}, '{unit}') => {line!r}")

emit(1, "ms")   # before: 'delay[1us] q[0];'    after: 'delay[1ms] q[0];'
emit(1, "ps")   # before: 'delay[1000ns] q[0];' after: 'delay[0.001ns] q[0];'

Test plan

• New TestQASM3ExporterRust.test_delay_units exercises every unit (ns, us, ms, s, dt, ps) with a subTest per case.
• Existing TestQASM3ExporterRust class (19 tests) still passes.
• Full test/python/qasm3/ suite (127 passed, 8 skipped) still passes.
• cargo check, cargo clippy -p qiskit-qasm3 -- -D warnings, and cargo fmt --check are clean.
• ruff check and black --check on test_export.py are clean.

AI tool disclosure

Per the Qiskit contribution policy: this change was prepared with assistance from Claude Opus 4.7 (1M context, via Claude Code). I reviewed every diff and verified all results locally; the model did not introduce any change I did not validate.

(Noting also that the originating issue #16097 was filed by @ihincks in collaboration with Claude 4.6.)

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