fix: Passing custom transforms to {web,turbo}pack

[timfish]

customTransforms was silently dropped by the webpack plugin - it only forwarded instrumentations and dcModule to the loader, so a config naming a transform threw per file and fell back to untransformed code.

• webpack: the plugin now forwards customTransforms. Webpack passes loader options by reference, so the functions arrive intact.
• Turbopack / thread-loader: these serialize options, so functions can't get through. New /webpack-loader-factory export — createLoader({ customTransforms }) builds a loader with the transforms bound at require time. Point the plugin at it with the new loaderPath option.
• Cache fix: webpack keys loaders by ruleset ident, not options content, so a changed config reused stale modules under cache: { type: 'filesystem' } - a pre-existing bug affecting instrumentations too. The ident is now derived from the config. cacheVersion covers data a transform reads but doesn't name, which Function.prototype.toString can't see.

Also updates the README's custom transform example to override tracingChannelImport so injection only happens where instrumentation actually ran — no more phantom Program configs.