feat(base-compile-testing): javac testing harness

[deer]

Introduces base-compile-testing, a JPMS-native replacement for google/compile-testing. The module provides a zero-dependency, in-memory javac harness for testing annotation processors and compiler-driven tooling.

API

• Compiler — immutable fluent builder wrapping javax.tools.JavaCompiler; supports withProcessors, withOptions, withClasspath, and withModulePath (a gap in Google's library)
• Compilation — Java record holding the compilation result: status, diagnostics (with errors(), warnings(), notes() stream accessors), and the full list of generated files with lookup helpers generatedSourceFile(String) and generatedFile(Location, String, String)
• JavaFileObjects — factory for in-memory source files via forSourceString and forSourceLines
• InMemoryJavaFileManager / ForwardingStandardJavaFileManager — internal plumbing that captures all compiler output in memory without touching the filesystem

Why not google/compile-testing? Google's library predates JPMS, pulls in Guava, and integrates with Truth rather than AssertJ. This module has no runtime dependencies beyond java.compiler, ships a proper module-info.java, and uses modern Java idioms throughout (records, Path-based paths, Writer.nullWriter() to suppress diagnostic noise on stderr).