refactor(java): replace static serializer spi lookup

[chaokunyang]

Why?

Follow up to #3679: static generated serializer lookup no longer needs aggregate SPI provider classes or service resources. The runtime can resolve generated serializers from the registered target class name, which simplifies Java/Kotlin generation and makes Android/R8 packaging rely on generated keep rules instead of broad user-written service-loader rules.

What does this PR do?

• Replaces StaticGeneratedSerializerProvider service-loader discovery with registry lookup based on escaped generated serializer class names and required constructors.
• Updates the Java annotation processor and Kotlin KSP output to use <target>_ForySerializer / <target>_ForyNativeSerializer names, emit per-target META-INF/proguard/ rules, and stop generating aggregate provider classes or service descriptors.
• Allows supported internal Kotlin @ForyStruct classes to generate internal serializers while keeping the constructor paths the Java runtime needs.
• Updates TypeResolver, static generated serializer registry tests, Kotlin KSP validation tests, and Kotlin xlang coverage for the new discovery path.
• Adds Android instrumentation coverage for static generated serializers and updates the Android CI setup to install the annotation processor artifact.
• Refreshes Java/Kotlin static generated serializer docs and removes the obsolete SPI discovery section from the xlang implementation guide.

Related issues

#3679

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

Not run; this PR changes static generated serializer lookup, generated metadata, docs, and tests rather than benchmarked serialization hot paths.