[Feature]: Add the sist Python package alongside Perl

[harryswift01]

Feature Request

Problem / Motivation

SIST has no Python package. master.pl shells out to the C++ qsidd binaries directly, and for cruciform/competition modes calls a second Perl script, IR_finder.pl, to compute candidate energies from an external irf report. A real Python implementation needs to exist and be validated before Perl can be removed.

Proposed Solution

• Add a src/sist/ Python package with a CLI mirroring master.pl's existing flags.
• Port IR_finder.pl's HTML report parsing and energy calculations to Python.
• Add orchestration that dispatches to the existing trans_three/trans_compete qsidd binaries the same way master.pl does today.
• Add unit tests for the ported energy and parsing functions, verified against the Perl source.
• Add an end-to-end test that runs the new sist CLI and compares its output against the existing reference baselines.
• Leave master.pl and IR_finder.pl as the installed entry point until the new package is proven equivalent.

Expected Impact

• No user-facing change, master.pl remains the shipped entry point.
• Establishes a Python implementation that can be verified against the existing scientific regression baselines before Perl is removed.