[GR-51209] Deduplicate strings image heap.

[graalvmbot]

• deduplicate Strings in JNINativeLinkage etc.
• add more options to -H:DumpHeap

AI Review

• The runtime CharSequence constructor now correctly rejects only image-build-time use, while allowing both JVM and image-runtime lookup paths.
• Hosted JNI registration routes through ofHosted(...), so deduplication happens on construction without exposing the hosted constructor.
• The normal after-image-write dump remains after feature callbacks; the relocatable-image early-exit path now emits a dump immediately after the write, before returning.
• build-end runs after layer archiving and final reporting.

Minimality and completeness

Ignoring the independently tracked ModuleDescriptor work, the changeset is concise and complete for its two stated goals: deduplicating the targeted JNI image-heap strings and adding final-build diagnostic dump points. The ofHosted(...) overloads add a small, justified API surface to keep hosted behavior explicit and the runtime path protected.