Add a \sites\ subcommand (list non-identity site positions)

[landau774]

Background

qsym-rs is a deliberately small symbolic Pauli-algebra engine and \qsym\ CLI (see \docs/design/qsym-rs.md). Its \PauliString\ (\src/lib.rs) already has \weight() -> usize\ (count of non-\I\ sites) and the \show\ subcommand displays \len\ and \weight\ as metadata. However, there is currently no way to ask which qubits a Pauli string acts on — i.e., the indices of its non-identity sites. This information is directly useful for identifying the support of an operator, e.g. when checking whether two operators act on disjoint qubit sets.

Task

Add a \sites() -> Vec\ method to \PauliString\ in \src/lib.rs\ that returns the 1-indexed positions of all non-\I\ sites. Then add a \sites\ subcommand to the \qsym\ CLI: \qsym sites . It must print the non-identity site positions as a comma-separated list on a single line, or print nothing (empty stdout) if the Pauli string is all identity.

Only \src/lib.rs\ and \src/main.rs\ change. Do not change any existing subcommands or signatures.

Verification

Run each command with \cargo run -q --\ (the crate's only binary), trimming trailing newlines before comparing stdout:

1. \cargo run -q -- sites XIZ\ — stdout is exactly \1, 3, exit code \