zipp-wasm: a persistent VM for browser hosts, and fat LTO

The WebAssembly embedding lands as a persistent VM that compiles once and re-enters later; fat LTO with one codegen unit buys about 2%; regex literals validated at compile time add 712 executions.

f6f4a804 adds zipp-wasm; 768287f8 adds the persistent-VM embedding API, "compile once, re-enter later", which is the model the browser Engine class still follows. 694886b8 turns on fat LTO and codegen-units = 1 (2.80× → 2.77×, "free"). 44c4c738 validates regular-expression literals at compile time, the single largest conformance jump of the week at +712 executions.