Stop terser from negating the coach IIFE's return value

[soulgalore]

The minified coach script is invoked in the browser via Selenium's
executeScript by way of return ${script}, so the outer IIFE's return
value IS the contract — sitespeed.io reads it back as
browserScriptsData.coach.coachAdvice and expects an object with the
collected advice, errors and version.

Terser's negate_iife compress option rewrites a top-level
(function(){…})() as !function(){…}() to shave a byte. That turned
the advice object into false on the wire. The bug was latent: an old
--compresss typo meant compression never ran, so the negation never
happened. Once that typo was fixed (dd178ab) compression kicked in and
sitespeed.io started seeing coachAdvice === false, then crashing the
DOM/HAR merge and the html templates that assume DOM data exists.

Disable negate_iife in the terser invocation so the IIFE keeps its
return value.

Co-authored-by: Claude Opus 4.7 (1M context) noreply@anthropic.com