fix(scala3): legalize JSON property names

[schani]

Summary

• generate legal, collision-free Scala identifiers for object properties instead of emitting raw JSON keys
• preserve original wire names with Circe configured codecs and uPickle @key annotations
• avoid inherited AnyRef/case-class member collisions such as clone and qualify generated runtime references that can be shadowed
• enable all six Scala JSON fixtures previously skipped for identifier limitations in both Circe and uPickle

This PR is stacked on #3078 so its expanded keywords.json fixture directly covers the clone regression. It can be retargeted to master after #3078 merges.

keyword-unions.schema remains skipped for an unrelated JVM restriction: its generated case class exceeds the 254-parameter limit.

Tests

• npm run build
• npm run test:unit
• npm run lint -- --no-errors-on-unmatched
• full QUICKTEST=true FIXTURE=scala3,scala3-upickle npm run test:fixtures (98 tests; all runtime tests passed; a JSON-vs-schema source diff for recursive.json was fixed and reverified)
• focused Circe/uPickle fixture runs for keywords.json, identifiers.json, simple-identifiers.json, blns-object.json, nst-test-suite.json, bug2521.json, and the C#: add --use-records option for C# 9+ record types #3071 record-member-names.json input