[skip] track used skips as class => [paths] map

[TomasVotruba]

Tracks used skips as a class => [path, path2] map instead of flat class|path join strings, matching the shape skips are already configured with in ->withSkip().

Before

ProcessResult::getUsedSkips() returned string[] where rule-scoped skips were encoded as "class|path", forcing the resolver to rebuild that join string to test membership:

if (! in_array($rectorClass . '|' . $path, $usedSkips, true)) {

After

getUsedSkips() returns array<string, string[]> — rule class to its matched paths — so the lookup is a direct nested check:

if (! in_array($path, $usedSkips[$rectorClass] ?? [], true)) {

Changes

• UsedSkipCollector::markUsed(string $skip, ?string $path = null) stores a nested map; provide() returns class => [paths].
• SkipSkipper marks the matched path under its rule instead of joining with |.
• ProcessResult constructor/addUsedSkips() validate and merge the nested shape per key.
• ParallelFileProcessor merges worker maps nested.
• Global path-only skips stay tracked by their own key (empty path set); resolver uses array_key_exists.

Tests updated accordingly. composer complete-check green.