Doc: LA sigrok fixture verification guide

[vailuc]

Background

Contributors now need to know how to add and verify their own sigrok-dumps fixtures for new LA protocols. The existing README is brief.

Goal

Expand the fixture README into a step-by-step verification guide.

Contents to add

• How to find a suitable capture in the public sigrok-dumps repository.
• How to update fetch-fixtures.sh to download the new fixture.
• How to inspect the fixture metadata (sample rate, channel labels).
• How to write a fixture test and determine the correct decoder config.
• How to run the smoke-test suite locally.

Acceptance criteria

• README explains the fixture lifecycle from discovery to test.
• A new contributor can follow the guide to add one fixture without asking for help.