Rename the two-qubit replacement-branch label prefix from *XX to ~XX or :replace:XX

[ciaranra]

Summary

p2_weights dictionaries for DEM building accept two-qubit Pauli labels such as XX as post-gate Pauli branches. A replacement branch, which omits the ideal two-qubit gate before applying the Pauli, was spelled with a * prefix (*XX, *II). The * symbol is being reserved for a different meaning in upcoming noise APIs, so replacement branches are now spelled ~XX or :replace:XX (and ~II / :replace:II for omission only).

Behavior change (breaking for configurations)

• ~XX and :replace:XX are accepted, equivalent, case-insensitive, and whitespace-trimmed.
• *XX is rejected with a ValueError that names both new spellings.
• Supplying both spellings for the same Pauli in one dictionary is rejected as a duplicate.
• Plain II remains rejected, and its message now points at ~II / :replace:II. That message was unreachable before, because plain II failed the label check first; it is reachable now.
• DEM builder behavior (PauliWeights::replacement_entries, approximation modes) is unchanged. Docs, diagnostics, and test messages in the DEM builder, the bindings, and the Python NoiseParameters docstrings use the new vocabulary.

Existing user configurations that use *XX will fail at construction with the migration message.

Tests

Rust, in the bindings parser: both spellings parse, case-insensitively; ~II parses as omission-only; *XX is rejected with a hint containing ~XX and :replace:XX; duplicate aliases are rejected; plain II names the new spellings. Python, in the surface-code noise-parameter tests: a ~XX replacement branch builds a DEM and samples; *XX raises ValueError mentioning ~XX.

Verification

• cargo test -p pecos-qec
• cargo test -p pecos-rslib p2_weight_parser_tests
• cargo clippy -p pecos-qec -p pecos-rslib --all-targets -- -D warnings
• cargo fmt -p pecos-qec -p pecos-rslib -- --check
• just python-ci-build-test, then pytest on the QEC surface and DEM test files: 338 passed, 18 xfailed
• just lint check

Context

Second of the separable changes ahead of the state-transition noise channel work.