Qiskit / qiskit / 34022270192

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QPY v18: replace InitialLayoutItemV2Pack with VirtualQBitPack enum (#16832)

* QPY v18: replace InitialLayoutItemV2Pack with VirtualQBitPack enum

Replace the `InitialLayoutItemV2Pack` struct with a proper `VirtualQBitPack` enum having
`Anonymous` and `InRegister { index, register_name }` variants. The old struct had two
fields — `index_value: i32` and `register_name_length: i32` — where `-1` in both meant
"anonymous qubit". These two fields were always in sync (both `-1` or both set) because
a qubit is either anonymous or in a register, never a mix. This dependency existed in the
data but was invisible in the type. The new enum makes it explicit: a qubit is either
`Anonymous` or `InRegister`, with no way to express an inconsistent state.

The wire encoding is version-dependent:
- v<=17 (backwards compatible): anonymous qubits encode as 2 x i32(-1) = 8 bytes, named
  qubits encode as i32(index) + i32(name_len) + name bytes.
- v>=18: anonymous qubits encode as 1 x i32(-1) = 4 bytes (the second sentinel i32 is
  dropped), named qubits are unchanged.

`VirtualQBitPack` implements `BinRead`/`BinWrite` manually with `type Args = (u8,)` (the
QPY version), and `LayoutV2Pack` forwards the version to each element via
`#[bw(args_raw = (version,))]`.

`dump_qpy` in `interface.rs` was calling `serialize(&pack_circuit(...))` — the no-args
variant. `serialize` requires `BinWrite::Args: Default`, so `QPYCircuit` was written with
`version=0`, which propagated down through `LayoutV2Pack` to every
`VirtualQBitPack::write_options` call. Since `version=0 < 18`, all anonymous qubits were
written with the old 8-byte encoding even when dumping a v18 file. The fix is to call
`serialize_with_args::<QPYCircuit, (u8,)>(..., (qpy_version,))` instead, which explicitly
passes the correct version through the entire write path down to each `VirtualQBitPack`.

Signed-off-by: Elad Venezian <eladv@il.ibm.com>

* QPY v18: add test asserting v18 is smaller than v17 for anonymous qubits

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