Add utility to find the start of a capture

[JoeyLemur]

Description

Adds ld-find-start, a CLI that scans an RF capture for sustained, advancing CAV/CLV VBI frame numbers and outputs a validated ld-decode --start N argument for the preceding clean pre-roll. It includes guarded handling for uncertain VBI-less captures, packaging integration, and unit tests.

Motivation

Rather than repeated guessing of where the start of the disc content is and reading through ld-decode errors, I wanted a command to automatically find the start.

Related Issues

This works under v7.2.1, but not under main 8af0932 -- see issue #1053

Changes Made

Added the ld-find-start command-line tool.

• Detects sustained sequential CAV/CLV VBI frame-address runs.
• Replays preceding fields to retain clean pre-roll before programme content.
• Reports uncertain VBI-less candidates without emitting an automatic --start.
• Added package entry-point integration and focused unit tests.

Testing

• python3 -m pytest -q — 25 passed.
• python3 -m pytest -q tests/test_start_finder.py — 12 passed.
• python3 -m isort --check-only and python3 -m py_compile for the new code.
• Manual CLI smoke tests: python3 ld-find-start --help and python3 -m lddecode.start_finder --version.

The unit tests cover CAV/CLV detection, rejected non-sequential addresses, decoder failures, clean pre-roll recovery, guarded fallback behavior, and CLI exit/output behavior.