engine-20260909-1605

The wasm backend's new execution model (patches/0006-wasm32-batch-chain-locals.patch, docs/engine-execution.md): blocks compile in batches of 64 after a warm-up with per-block import relocation, compiled blocks tail-call their successors (dispatcher fallback on browsers without tail calls), guest registers in wasm locals. opencode --version 2.4-2.9x faster; hello/ripgrep/jujutsu/python 1.0-1.3x; no regressions in the exec-bench suite. Same snapshot and guest image as engine-20260907-0100.