c2-sea-of-nodeslisted

# C2 and the Sea-of-Nodes IR ## Purpose Decide _why_ a compilation came out the way it did, from the mechanism rather than from folklore. The failure this prevents is the confident non-fix: a runbook that sets `-XX:CompileThreshold` under tiered compilation and changes nothing at all, or a refactoring done because "the JIT will optimise it" when the JIT never performs algorithmic changes. Start with tier/version, call-site inlining and escape state, then follow the failing transformation: type/profile stability, alias/memory dependencies, loop/range checks, vectorization, macro expansion, matching, scheduling and register pressure can each own the result. The phase model routes evidence; it is not a three-question completeness claim. ## Workflow The authoring baseline is HotSpot C2 on JDK 25 (reference measurements use Temurin 25.0.3); these are implementation details, not Java language guarantees. Before running recipes, inspect the project's toolchain, CI/runtime image and actual `java -version`, including vendor/update, product versus debug build, compiler and effective flags. Level 4 can be JVMCI rather than C2; route that compiler's internals to `graalvm-jit`. Do not upgrade the target runtime or enable preview features to match this baseline. For another release, check `java -Xlog:help` and flag availability first; unavailable diagnostics require an alternative evidence source, not an assumed result. 1. **Establish the compilation history before anything else.** R