Fail builds on unused CSS by default

[gregjotau]

Summary

• add one cacheable application-wide thimCssUsageCheck task and make check/thimCheck fail on unused first-party CSS by default
• scan production HTML, JVM, JavaScript, and TypeScript sources across every Gradle module, with exact runtime-class allowlisting and explicit module opt-out
• make unused fragment parameters errors under the existing strict fragment default
• bump the release to 0.9.0 and document the strict CSS behavior

Consumer proof

• ReAI: found 14 dead classes; remaining 838 definitions resolve as 831 exact + 7 dynamic-prefix uses
• Utin: found 2 dead component classes; remaining 1039 definitions resolve as 1027 exact + 12 dynamic-prefix uses
• Eteo: 215/215 definitions used
• Ecomtools: no first-party CSS in the conventional source root

Verification

• ./gradlew clean build --configuration-cache
• configuration-cache replay
• composite thimCheck in ReAI, Utin, Eteo, and Ecomtools
• composite thimCssUsageCheck after removing verified dead CSS