Document improvement hotspots and cross-target warning fixes

[Copilot]

The repo needed clear, actionable areas of improvement. Added fresh observations to guide the next iteration.

• Fresh observations (Dec 19, 2025): Documented steps to silence cross-target LLVM warnings and prioritize coverage on low-hit modules.
• Actionable focus: Highlighted runtime bitcode packaging and target-data handling as quick wins; noted coverage hotspots for compiler/runtime.

Example excerpt:

## Fresh Observations (December 19, 2025)
- Silence cross-target warnings by packaging wasm32/aarch64 target data; ship runtime bitcode in build artifacts.
- Raise coverage in compiler.codegen.llvm_backend and runtime/clarity to reduce noisy gaps.

Original prompt

Look for areas of improvement

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