Reduce locking in HostClassLoader

[hubertp]

Pull Request Description

Using a neat trick described in #13219 (comment) to avoid large synchronization blocks.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.

[Akirathan]

This feels like something that could (should) be unit tested:

• Create an isolated HostClassLoader in the unit test
• Point it to some arbitrary jar archives / directories with classes
  • Scan from classpath system property, pass via sbt, ...
• Spawn multiple threads that will try to load same classes in parallel.

Or alternatively, move the scheduling/synchronization logic outside HostClassLoader and test it in isolation.

Let's try hard to ensure that we do not introduce hard-to-debug race conditions.

[enso-bot]

Hubert Plociniczak reports a new STANDUP for the provided date (2025-08-19):

Progress: Addressing some comments #13781, improving locking in HostClassLoader in #13807. Added draft of caching of all arguments (#13813) which appears to have improved startup but will need more testing. It should be finished by 2025-08-19.

Next Day: Next day I will be working on the #10525 task. Pick up work on reactive caches