feat: add soft-reference compile-time caches

[ningyang1988]

This PR implements the soft-reference compile-time cache requested in #350 with a small, targeted patch.

What changed

• Scala 2: wraps compile-time caches for ref, fullDb, and inheritanceDb with SoftReference
• Scala 3: adds the corresponding soft-reference compile-time caches in TypeInspections
• Keeps the patch limited to 2 source files only

Validation

• Scala 3 TagTest: passed
• Scala 2 TagTest: passed
• cache disabled check: -Dizumi.reflect.rtti.cache.compile=false still passes TagTest
• full regression: sbt +clean +test passed (209 tests, 0 failed)

Notes

• I intentionally did not include unrelated changes
• I also did not add benchmark claims from weak single-run samples; this PR focuses on requested behavior and regression safety

Closes #350

[CLAassistant]

All committers have signed the CLA.

[ningyang1988]

CLA assistant shows that the agreement has already been accepted on my side ('You have agreed to the CLA for zio/izumi-reflect'), but the PR check is still pending as not signed. I also retriggered the recheck link while logged into the same GitHub account that opened this PR. Could you please help re-sync or re-run the CLA check? Thanks!

[ningyang1988]

Gentle bump — checking if there is anything I can do to help move this PR forward. Happy to address any feedback. Thanks!