test(fscache): add filesystem limits tests and benchmarks

[bartventer]

Add comprehensive testing for long URL handling to validate fragmentation behavior discussed in PR #17.

• Test URL lengths up to 100KB
• Benchmark performance scaling
• Verify directory depth handling

Related to #16, addresses concerns from PR #17

[bartventer]

Seems like windows is the culprit, let me investigate...

[bartventer]

The fragmentation feature added in v0.11.0 might be a regression, it may have
been implemented for all platforms when the original issue was Windows-specific.
Going to run some more tests to verify.

[bartventer]

The fragmentation feature added in v0.11.0 might be a regression, it may have been implemented for all platforms when the original issue was Windows-specific. Going to run some more tests to verify.

Update: Additional benchmarks confirm fragmentation is NOT a regression. Base64 fails
on Unix with "file name too long" for long keys, while fragmentation works
cross-platform. Performance overhead is acceptable vs complete failure.