Cache optimized default searches

[OskarEichler]

Summary

Reuse a bounded, thread-safe default CachingRuntime for optionless JMESPath.search calls and cache optimized expression trees.

This implements the maintainer direction recorded in #25 while preserving custom runtime options and the existing Runtime/CachingParser APIs.

Reproduction

The current module-level helper constructs a new runtime/parser and re-optimizes on every call. A parser-count model proves one parse across repeated and concurrent default searches; nonempty runtime options still get isolated runtimes.

Verification

• isolated branch: 1,790 examples, zero failures
• 803 repeated/concurrent cache and option-isolation checks
• cumulative candidate: 892 current compliance checks, zero failures
• 20,000 repeated representative searches: 0.388s baseline vs 0.152s candidate (about 2.6x faster locally)
• all runtime syntax, 33-path package build and Rails 8.1.3.1 integration pass

Compatibility

Calls with runtime options remain uncached and preserve their parser/error configuration. Cache size follows the existing 1,000-entry policy; optimized nodes are immutable during visits. Prepared with AI-assisted source review; no repository tests were changed.