WIP: Bust pin cache when solargraph's own lib code changes

[apiology]

Summary

• PinCache.work_dir keys the on-disk cache directory only on Solargraph::VERSION, a hardcoded string. Marshal.load silently tolerates deserializing pins that were serialized by an older Pin::Callable definition with fewer ivars (missing ones just read back as nil), so a gem source swap that changes Pin behavior without bumping VERSION can leave stale, schema-mismatched pins cached on disk after upgrading.
• Flagged via Fix @generic return type lost when method also declares a block param castwide/solargraph#1274 (comment): a consumer bisected a reported regression by swapping their solargraph gem revision (git-sourced, same VERSION before/after) without clearing ~/.cache/solargraph. That gave Pin::Callable#block_required? == false (nil read back from the old-schema cached pin) for an RBS signature whose block is genuinely required, letting a generic-returning block overload spuriously match a call site with no block and leak an unresolved generic<X> into the inferred return type.
• I reproduced this precisely: populate the pin cache on the pre-Fix @generic return type lost when method also declares a block param castwide/solargraph#1274 commit, switch to the post-Fix @generic return type lost when method also declares a block param castwide/solargraph#1274 commit under the same forced VERSION, don't clear the cache -> same symptom on the same minimal repro from the linked comment. Clearing the cache (or this fix) makes it go away.
• Fix: fold a digest of solargraph's own lib/ file mtimes+sizes into work_dir, so any code change -- not just a VERSION bump -- busts the cache automatically.

Marked draft/WIP: this is a real, reproduced bug, but low urgency (self-resolves once affected users clear their cache once) -- opening now mainly so the finding and fix aren't lost.

Test plan

• bundle exec rspec spec/pin_cache_spec.rb -- new spec covers digest sensitivity and that it's folded into work_dir
• bundle exec rspec -- full suite green (1625 examples, 0 failures, 60 pending)
• bundle exec rubocop lib/solargraph/pin_cache.rb spec/pin_cache_spec.rb -- clean
• bundle exec solargraph typecheck --level strong lib/solargraph/pin_cache.rb -- no new problems on modified lines
• Manually reproduced the stale-cache regression from the linked comment and confirmed this fix resolves it

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